KVM: arm64: gic-v5: Probe for GICv5

Add in a probe function for GICv5 which enables support for GICv3
guests on a GICv5 host, if FEAT_GCIE_LEGACY is supported by the
hardware.

Co-authored-by: Timothy Hayes <timothy.hayes@arm.com>
Signed-off-by: Timothy Hayes <timothy.hayes@arm.com>
Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Link: https://lore.kernel.org/r/20250627100847.1022515-6-sascha.bischoff@arm.com


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>