dt-bindings: display: add devicetree bindings for pxa300-gcu

This patch adds the binding documentation for the pxa300 gcu.
Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>

Documentation/devicetree/bindings/display/marvell,pxa300-gcu.txt

+PXA3xx GCU Controller
+---------------------
+
+Required properties:
+ - compatible : "marvell,pxa300-gcu"
+ - reg : should contain the register range (address and length).
+ - interrupts : Controller interrupt.
+ - clocks: phandle to the PXA specific input clock.
+
+Example for PXA300:
+
+	display-controller@54000000 {
+		compatible = "marvell,pxa300-gcu";
+		reg = <0x54000000 0x1000>;
+		interrupts = <39>;
+		clocks = <&clks CLK_PXA300_GCU>;
+	};