arch-arm: Add ID_MMFR4{,EL1} system registers

arch-arm: Add ID_MMFR4{,EL1} system registers

Change-Id: Id50ebd2ef2e69ecbd3b7f64a4e9eafe00e283806
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34876
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>