irqchip: gic-v2m: Add support for ARM GICv2m MSI(-X) doorbell

ARM GICv2m specification extends GICv2 to support MSI(-X) with
a new register frame. This allows a GICv2 based system to support
MSI with minimal changes.

Signed-off-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
[maz: converted the driver to use stacked irq domains,
      updated changelog]
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1416941243-7181-2-git-send-email-marc.zyngier@arm.com


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