include arm64: Add mm.h to unify mmu and mpu

The arm_mmu.h and arm_mpu.h have some common logic. To reduce some
redundency and the ugly 'if defined' marcos, I add a new header
include/arch/arm64/mm.h to include arm_mmu.h or arm_mpu.h depending on
CONFIG_ARM_MMU or CONFIG_ARM_MPU. Thus, in the future, the common code
of mmu and mpu should be in include/arch/arm64/mm.h.

Signed-off-by: Jaxson Han <jaxson.han@arm.com>