tests: arch: arm: verify Armv8.1-M MPU PXN

Adds a test to verify Armv8.1-M MPU architecture's PXN attribute.

Verify PXN via static mpu config by utilizing existing __ramfunc MPU region
Since previous commit changes the behaviour of __ramfunc region,
executing userspace code in this region in privileged mode will
result in an MPU fault while execution in unprivileged mode should
work fine.

Verify PXN via DT by creating a custom section in the unused SRAM region.
Since the overlay sets the PXN attribute for the SRAM, executing code
in this region in privileged mode will result in an MPU fault while,
execution in unprivileged mode should work fine.
Also, instead of testing DT config for each board, do it only for MPS3
boards that which can be tested with simulation.

Signed-off-by: Sudan Landge <sudan.landge@arm.com>