arch: arm: mpu: remove unnecessary MPU region type definitions

This commit removes the unnecessary MPU region type definitions
from arm_core_mpu_dev.h, as they are not used any more in any of
the architecture-specific MPU implementations (ARMv7-M, NXP, and
ARMv8-M MPU).

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