memory address over 0x2000_7ffc is not accessible in mps2-an505

I currently run qemu with the following options
`qemu-system-aarch64 -machine mps2-an505 -cpu cortex-m33 -m 16`

For some reason, memory address over 0x2000_7ffc is not accessible.
It can be tested in gdb as follow.

(gdb) x/x 0x20007ffc
0x20007ffc:	0x00000000
(gdb) x/x 0x20007ffd
0x20007ffd:	Cannot access memory at address 0x20007ffd
(gdb) x/x 0x20008000
0x20008000:	Cannot access memory at address 0x20008000