arch: arm: add compile-time guards for arm_mpu code
arch: arm: add compile-time guards for arm_mpu code

This commit places all the implementation in arm_mpu.c within
build-time guards, ensuring that it is compiled-in only if the
kernel features requiring MPU are present (i.e. USERSPACE,
APPLICATION_MEMORY, or MPU_STACK_GUARDS). An exception to that
is the code required for MPU initialization, which is left out,
as MPU initialization occurs always if MPU is present.

Signed-off-by: Ioannis Glaropoulos <5921cc8bab7e1d4329f52fd8f6268f9692e3de80@nordicsemi.no>

nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,explora26/zephyr,ldts/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,galak/zephyr,galak/zephyr,punitvara/zephyr,punitvara/zephyr,finikorg/zephyr,punitvara/zephyr,finikorg/zephyr,kraj/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,kraj/zephyr,punitvara/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,ldts/zephyr,ldts/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,kraj/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr