dev-arm: Define enum masks for SMMU_CR0 register

dev-arm: Define enum masks for SMMU_CR0 register

The configuration register is a vital register in the SMMU, and using
enum masks will make the code more readable/understandable

Change-Id: Ia117db56c457fe876ae38be391c386e502f34384
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Michiel Van Tol <michiel.vantol@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19632
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>