dev-arm: Cleanup GICv3 initialization

dev-arm: Cleanup GICv3 initialization

This patch is removing the unnecessary initState() / reset() methods
from GICv3 classes, since we can initialize everything at
construction/init time

Change-Id: Ia70edcc4ca4f11878fac0024342e4f2cd81883a0
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20636
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>