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 <Ioannis.Glaropoulos@nordicsemi.no>