arm64/sysreg: Add ICH_VCTLR_EL2

This system register is required to enable/disable V3 legacy mode when
running on a GICv5 host.

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/20250627100847.1022515-4-sascha.bischoff@arm.com


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