irqchip/gic: Split vGIC probing information from the GIC code

The vGIC advertising code is unsurprisingly very much tied to
the GIC implementations. However, we are about to extend the
support to lesser implementations.

Let's dissociate the vgic registration from the GIC code and
move it into KVM, where it makes a bit more sense. This also
allows us to mark the gic_kvm_info structures as __initdata.

Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>