Documentation: Add device tree binding for Goldfish FB driver

Add documentation for DT binding of Goldfish FB driver. The compatible
string used by OS for binding the driver is "google,goldfish-fb".
Signed-off-by: NMiodrag Dinic <miodrag.dinic@mips.com>
Signed-off-by: NGoran Ferenc <goran.ferenc@mips.com>
Signed-off-by: NAleksandar Markovic <aleksandar.markovic@mips.com>
Acked-by: NRob Herring <robh@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Douglas Leung <douglas.leung@mips.com>
Cc: James Hogan <james.hogan@mips.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Petar Jovanovic <petar.jovanovic@mips.com>
Cc: Raghu Gandham <raghu.gandham@mips.com>
Signed-off-by: NBartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>

Documentation/devicetree/bindings/display/google,goldfish-fb.txt

+Android Goldfish framebuffer
+
+Android Goldfish framebuffer device used by Android emulator.
+
+Required properties:
+
+- compatible : should contain "google,goldfish-fb"
+- reg        : <registers mapping>
+- interrupts : <interrupt mapping>
+
+Example:
+
+	display-controller@1f008000 {
+		compatible = "google,goldfish-fb";
+		interrupts = <0x10>;
+		reg = <0x1f008000 0x100>;
+	};