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>
Git-commit: b3a92e2c4441affceca9e05905723532e4a61e4d
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Puja Gupta <pujag@codeaurora.org>