arm64/sysreg: Add ICH_VMCR_EL2

Add the ICH_VMCR_EL2 register, which is required for the upcoming
GICv5 KVM support. This register has two different field encodings,
based on if it is used for GICv3 or GICv5-based VMs. The
GICv5-specific field encodings are generated with a FEAT_GCIE prefix.

This register is already described in the GICv3 KVM code
directly. This will be ported across to use the generated encodings as
part of an upcoming change.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>