dt-bindings: add GIC-400 binding

Add "arm,gic-400" compatible property for ARM GIC-400 IP.
Signed-off-by: NRob 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: NKumar Gala <galak@codeaurora.org>

Documentation/devicetree/bindings/arm/gic.txt

@@ -11,6 +11,7 @@ have PPIs or SGIs.
 Main node required properties:
 
 - compatible : should be one of:
+	"arm,gic-400"
 	"arm,cortex-a15-gic"
 	"arm,cortex-a9-gic"
 	"arm,cortex-a7-gic"