dt-bindings: Documents the mbigen bindings

Add the mbigen msi interrupt controller bindings document.

This patch based on Mark Rutland's patch
https://lkml.org/lkml/2015/7/23/558



Signed-off-by: Ma Jun <majun258@huawei.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>