irqchip: gic-v2m: Add DT bindings for GICv2m

Update the GIC DT bindings to support GICv2m.

Signed-off-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
[maz: split DT patch from main driver, updated changelog]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1416941243-7181-3-git-send-email-marc.zyngier@arm.com


Signed-off-by: Jason Cooper <jason@lakedaemon.net>