dt-bindings: add GIC-400 binding

Add "arm,gic-400" compatible property for ARM GIC-400 IP.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Acked-by: Kumar Gala <galak@codeaurora.org>