arch: arm: mpu: group together USERSPACE-related implementation

This commit moves all internal implementation, relevant for
User mode, in a single place in the arm_mpu_v7_internal.h,
arm_mpu_v8_internal, and in arm_mpu.c. Additionally, the
commit cleans up internal function _get_region_attr() that
is not used any more.

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