dev-arm: Reapply GICv3 changes that were lost during refactoring

dev-arm: Reapply GICv3 changes that were lost during refactoring

The GICv3 code refactoring performed by:

https://gem5-review.googlesource.com/c/public/gem5/+/16484

reverted the following patches

https://gem5-review.googlesource.com/c/public/gem5/+/16544
https://gem5-review.googlesource.com/c/public/gem5/+/16545/3

This commit is reintroducing them

Change-Id: I2c875c11570ed66ec9203449446faca3864c64d6
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19229
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>