arch: arm: Add PXN attr for userspace MPU regions

What is the change?
 - PXN attributes is set for static mpu regions such that
   __ramfunc and __ram_text_reloc having userspace code
   cannot be executed in privileged mode.
 - Updated the notable change section to inform about the change in
   behaviour of code executed from __ramfunc and __ram_text_reloc MPU
   regions.

Why do we need this change?
 - The current static MPU regions allows executing userspace/unprivileged
   code from privileged mode which may not be expected and can lead to
   secure privileged escalation attacks.

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