armv8: Add ARMv8 MPU configuration logic

Armv8r64 is the first Armv8 platform that only has a PMSA at the
current exception level. The architecture supplement for Armv8r64
describes new fields in ID_AA64MMFR0_EL1 which can be used to detect
whether a VMSA or PMSA is present. These fields are RES0 on Armv8a.

Add logic to read these fields and, for the protection of the memory
used by U-Boot, initialize the MPU instead of the MMU during init, then
clear the MPU regions before transition to the next stage.

Provide a default (blank) MPU memory map, which can be overridden by
board configurations.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>