ARM: new VGIC: vgic-init: register VGIC

ARM: new VGIC: vgic-init: register VGIC

This patch implements the function which is called by Xen when it wants
to register the virtual GIC.
This also implements vgic_max_vcpus() for the new VGIC, which reports
back the maximum number of VCPUs a certain GIC model supports. Similar
to the counterpart in the "old" VGIC, we return some maximum value if
the VGIC has not been initialised yet.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>