dev-arm: GICv3, handle GICR_ICFGR0 WI behaviour

dev-arm: GICv3, handle GICR_ICFGR0 WI behaviour

Architecture states write accesses to GICR_ICFGR0 are WI. This patch
implements handling of this behaviour instead of crashing as an invalid
offset. This is required to support certain software behaviour.

Change-Id: I1f8c57838566c360d243a925306ec35c64a920b2
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24063
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>