arch: arm: mpu: remove arm_core_mpu_enable/disable internal API
arch: arm: mpu: remove arm_core_mpu_enable/disable internal API

arm_core_mpu_enable() and arm_core_mpu_disable() functions are
effectively static functions, only used in the drivers for ARM
and NXP MPU, therefore, we do not need to expose them in the
arm_core_mpu_dev.h header.

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

finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr