irqchip: GICv3: Binding updates for ITS

Add the documentation for the bindings describing the GICv3 ITS.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1416839720-18400-14-git-send-email-marc.zyngier@arm.com


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