ARM: dts: Add Samsung G2D DT bindings documentation

Added documentaion about G2D bindings.

Cc: Tomasz Figa <tomasz.figa@gmail.com>
Cc: Inki Dae <inki.dae@samsung.com>
Signed-off-by: NSachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: NKukjin Kim <kgene.kim@samsung.com>

Documentation/devicetree/bindings/gpu/samsung-g2d.txt

+* Samsung 2D Graphics Accelerator
+
+Required properties:
+  - compatible : value should be one among the following:
+	(a) "samsung,s5pv210-g2d" for G2D IP present in S5PV210 & Exynos4210 SoC
+	(b) "samsung,exynos4212-g2d" for G2D IP present in Exynos4x12 SoCs
+	(c) "samsung,exynos5250-g2d" for G2D IP present in Exynos5250 SoC
+
+  - reg : Physical base address of the IP registers and length of memory
+	  mapped region.
+
+  - interrupts : G2D interrupt number to the CPU.
+
+Example:
+	g2d@12800000 {
+		compatible = "samsung,s5pv210-g2d";
+		reg = <0x12800000 0x1000>;
+		interrupts = <0 89 0>;
+		status = "disabled";
+	};