arm/mpu: Introduce MPU memory region map structure
Introduce pr_t typedef which is a structure having the prbar and prlar members,
each being structured as the registers of the AArch32 Armv8-R architecture.
Also, define MPU_REGION_RES0 to 0 as there are no reserved 0 bits beyond the
BASE or LIMIT bitfields in prbar or prlar respectively.
In pr_of_addr(), enclose prbar and prlar arm64 specific bitfields with
appropriate macros. So, that this function can be later reused for arm32 as
well.