arm: mpu: Unify ARM_MPU kconfig definitions

Previously, there were two separate Kconfig definitions
of ARM_MPU located at:
- arch/arm/core/mpu/Kconfig
- arch/arm64/core/cortex_r/Kconfig

This lead to incomplete default settings and unexpected
missing configurations.

This commit combines the two into a single unified definition
now located at arch/common/Kconfig to prevent this.

Signed-off-by: Samuel Chee <samche01@arm.com>