dev-arm: Implement GIC-400 model from GicV2

dev-arm: Implement GIC-400 model from GicV2

Implementation registers for the GICv2 model currently hold values
referring to a GIC-400 implementation. This patch is making them
parametrizable so that it is possible to instantiate a GIC-400 model.
The patch is also modifying Realview platform to use new GIC-400 model
in lieau of GICv2.

Change-Id: I446db8c796ee3c2708af91e9139f0a6e7947321b
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/15277
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>