dev-arm: Add GICv4 extension switch in GICv3

dev-arm: Add GICv4 extension switch in GICv3

This is currently used only for determining which is the correct
size of redistributors in memory (256KB in GICv4 and 128KB in GICv3)

Change-Id: I2c07005e97167fde03548313c9927176788f31dd
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/+/18391
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>