ARM: VGIC: wire new VGIC(-v2) files into Xen build system

ARM: VGIC: wire new VGIC(-v2) files into Xen build system

Now that we have both the old VGIC prepared to cope with a sibling and
the code for the new VGIC in place, lets add a Kconfig option to enable
the new code and wire it into the Xen build system.
This will add a compile time option to use either the "old" or the "new"
VGIC.
In the moment this is restricted to a vGIC-v2. To make the build system
happy, we provide a temporary dummy implementation of
vgic_v3_setup_hw() to allow building for now.

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