ARM: DT: Add binding for GIC virtualization extentions (VGIC)

The GICv2 can have virtualization extension support, consisting
of an additional set of registers and interrupts. Add the necessary
binding to the GIC DT documentation.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>