xen/arm32: Create the same boot-time MPU regions as arm64

xen/arm32: Create the same boot-time MPU regions as arm64

Create Boot-time MPU protection regions (similar to Armv8-R AArch64) for
Armv8-R AArch32.
Also, defined *_PRBAR macros for arm32. The only difference from arm64 is that
XN is 1-bit for arm32.
Define the system registers and macros in mpu/cpregs.h.

Introduce WRITE_SYSREG_ASM() to write to system registers in assembly.

Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>
Tested-by: Luca Fancellu <luca.fancellu@arm.com>