documentation: dt: omap: crossbar: Add description for interrupt consumer

The current crossbar description does not include the description
required for the consumer of the crossbar, a.k.a devices whoes events
pass through the crossbar into the GIC interrupt controller.

So, provide documentation for the same.
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Sricharan R <r.sricharan@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Link: https://lkml.kernel.org/r/1403766634-18543-16-git-send-email-r.sricharan@ti.comSigned-off-by: Jason Cooper <jason@lakedaemon.net>

Documentation/devicetree/bindings/arm/omap/crossbar.txt

@@ -36,3 +36,20 @@ Examples:
 			ti,irqs-reserved = <0 1 2 3 5 6 131 132 139 140>;
 			ti,irqs-skip = <10 133 139 140>;
 		};
+
+Consumer:
+========
+See Documentation/devicetree/bindings/interrupt-controller/interrupts.txt and
+Documentation/devicetree/bindings/arm/gic.txt for further details.
+
+An interrupt consumer on an SoC using crossbar will use:
+	interrupts = <GIC_SPI request_number interrupt_level>
+request number shall be between 0 to that described by
+"ti,max-crossbar-sources"
+
+Example:
+	device_x@0x4a023000 {
+		/* Crossbar 8 used */
+		interrupts = <GIC_SPI 8 IRQ_TYPE_LEVEL_HIGH>;
+		...
+	};