arch: arm: add Cortex-M MPU context save/restore API

Add struct z_mpu_context_retained and parameterized functions
z_arm_save_mpu_context() and z_arm_restore_mpu_context() to allow
saving and restoring MPU configuration to/from retained RAM.

This enables preserving MPU state across deep sleep or power-down
modes on Cortex-M devices.

The API is exposed under include/zephyr/arch/arm/mpu/arm_mpu.h
and implemented in arch/arm/core/mpu/arm_mpu.c.

Signed-off-by: Michele Sardo <msmttchr@gmail.com>