arch-arm: Implement AArch64 ID_AA64MMFR2_EL1 register

arch-arm: Implement AArch64 ID_AA64MMFR2_EL1 register

This patch implements AArch64 Memory Model Feature Register 2
(from ARMv8.2)

Change-Id: I16d9acaf620fac6d1206e208bd143daec1657daf
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/13066
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>