buffer overflow in aspeed gpio
Description of problem:
The following log reveals it:

```
==2602930==ERROR: AddressSanitizer: global-buffer-overflow on address 0x55a5da29e128 at pc 0x55a5d700dc62 bp 0x7fff096c4e90 sp 0x7fff096c4e88
READ of size 2 at 0x55a5da29e128 thread T0
    #0 0x55a5d700dc61 in aspeed_gpio_read /home/joey/repo/qemu/build/../hw/gpio/aspeed_gpio.c:564:14
    #1 0x55a5d933f3ab in memory_region_read_accessor /home/joey/repo/qemu/build/../system/memory.c:445:11
    #2 0x55a5d92fba40 in access_with_adjusted_size /home/joey/repo/qemu/build/../system/memory.c:573:18
    #3 0x55a5d92f842c in memory_region_dispatch_read1 /home/joey/repo/qemu/build/../system/memory.c:1426:16
    #4 0x55a5d92f7b68 in memory_region_dispatch_read /home/joey/repo/qemu/build/../system/memory.c:1459:9
    #5 0x55a5d9376ad1 in flatview_read_continue_step /home/joey/repo/qemu/build/../system/physmem.c:2836:18
    #6 0x55a5d9376399 in flatview_read_continue /home/joey/repo/qemu/build/../system/physmem.c:2877:19
    #7 0x55a5d93775b8 in flatview_read /home/joey/repo/qemu/build/../system/physmem.c:2907:12
    #8 0x55a5d9377078 in address_space_read_full /home/joey/repo/qemu/build/../system/physmem.c:2920:18
    #9 0x55a5d8189aa2 in address_space_read /home/joey/repo/qemu/include/exec/memory.h:3100:18
    #10 0x55a5d8189aa2 in qtest_process_command /home/joey/repo/qemu/build/../system/qtest.c:597:13
    #11 0x55a5d818231d in qtest_process_inbuf /home/joey/repo/qemu/build/../system/qtest.c:811:9
    #12 0x55a5d81915ae in qtest_read /home/joey/repo/qemu/build/../system/qtest.c:823:5
    #13 0x55a5d9bc115d in qemu_chr_be_write_impl /home/joey/repo/qemu/build/../chardev/char.c:214:9
    #14 0x55a5d9bc1219 in qemu_chr_be_write /home/joey/repo/qemu/build/../chardev/char.c:226:9
    #15 0x55a5d9bccd25 in fd_chr_read /home/joey/repo/qemu/build/../chardev/char-fd.c:72:9
    #16 0x55a5d95d958c in qio_channel_fd_source_dispatch /home/joey/repo/qemu/build/../io/channel-watch.c:84:12
    #17 0x7f8909babc43 in g_main_context_dispatch (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x55c43)
    #18 0x55a5d9f62319 in glib_pollfds_poll /home/joey/repo/qemu/build/../util/main-loop.c:287:9
    #19 0x55a5d9f60c53 in os_host_main_loop_wait /home/joey/repo/qemu/build/../util/main-loop.c:310:5
    #20 0x55a5d9f6081c in main_loop_wait /home/joey/repo/qemu/build/../util/main-loop.c:589:11
    #21 0x55a5d8198807 in qemu_main_loop /home/joey/repo/qemu/build/../system/runstate.c:796:9
    #22 0x55a5d9544c6c in qemu_default_main /home/joey/repo/qemu/build/../system/main.c:37:14
    #23 0x55a5d9544cb7 in main /home/joey/repo/qemu/build/../system/main.c:48:12
    #24 0x7f8909229d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #25 0x7f8909229e3f in __libc_start_main csu/../csu/libc-start.c:392:3
    #26 0x55a5d671ed34 in _start (/home/joey/repo/qemu/build/qemu-system-aarch64+0x2773d34)

0x55a5da29e128 is located 24 bytes to the left of global variable '<string literal>' defined in '../hw/gpio/aspeed_gpio.c:1180:23' (0x55a5da29e140) of size 20
  '<string literal>' is ascii string 'aspeed.gpio-ast2500'
0x55a5da29e128 is located 22 bytes to the right of global variable '<string literal>' defined in '/home/joey/repo/qemu/include/hw/gpio/aspeed_gpio.h:17:1' (0x55a5da29e100) of size 18
  '<string literal>' is ascii string 'ASPEED_GPIO_CLASS'
SUMMARY: AddressSanitizer: global-buffer-overflow /home/joey/repo/qemu/build/../hw/gpio/aspeed_gpio.c:564:14 in aspeed_gpio_read
Shadow bytes around the buggy address:
  0x0ab53b44bbd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ab53b44bbe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ab53b44bbf0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ab53b44bc00: 00 00 00 00 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
  0x0ab53b44bc10: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
=>0x0ab53b44bc20: 00 00 02 f9 f9[f9]f9 f9 00 00 04 f9 f9 f9 f9 f9
  0x0ab53b44bc30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ab53b44bc40: 00 00 00 00 00 00 00 00 f9 f9 f9 f9 00 00 04 f9
  0x0ab53b44bc50: f9 f9 f9 f9 00 00 00 01 f9 f9 f9 f9 00 00 00 00
  0x0ab53b44bc60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ab53b44bc70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
```
Steps to reproduce:
```
cat << EOF | qemu-system-aarch64 -display \
none -machine accel=qtest, -m 512M -machine ast1030-evb -qtest stdio
readq 0x7e780272
EOF
```