MPS2 AN521 has the wrong number of MPU region defined
Description of problem:
The AN521 is integrating SSE-200 on the MPS2+ FPGA prototyping board.
The current implementation in qemu behaves as though there are 16MPU regions when it really only has 8, as describes as `MPU_NS` and `MPU_S` core configuration parameters in the SSE-200's [Techincal Reference Manual](https://developer.arm.com/documentation/101104/0200/functional-description/cpu-elements/cortex-m33-configurations?lang=en).
Steps to reproduce:
1. Prepare your Zephyr dev environment
2. fix `boards/arm/mps2_an521/mps2_an521.dts` to set `arm,num-mpu-regions`  to the appropriate value of 8.
3. build a Zephyr test such as `west build -p -b mps2_an521 -T tests/kernel/interrupt/arch.interrupt` 
4. run `qemu-system-arm -machine mps2-an521 -chardev stdio,id=con,mux=on -serial chardev:con -kernel ./build/zephyr/zephyr.elf`
Additional information:
With matching MPU region number in QEMU and Zephyr's DTS, the application shows the test suite's progress & outcome.
If there's a mismatch, the application will enter a fault and not display the expected traces.