dt-bindings: add document for Rockchip RGA module

RGA is a separate 2D raster graphic acceleration unit.

Change-Id: I510a4799e6c69afe01b2f2adfd6be84e322ff9f2
Signed-off-by: Yakir Yang <ykk@rock-chips.com>

diff --git a/Documentation/devicetree/bindings/display/rockchip/rockchip-rga.txt b/Documentation/devicetree/bindings/display/rockchip/rockchip-rga.txt
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+device-tree bindings for rockchip 2D raster graphic acceleration controller (RGA)
+
+RGA is a separate 2D raster graphic acceleration unit. It accelerates 2D
+graphics operations, such as point/line drawing, image scaling, rotation,
+BitBLT, alpha blending and image blur/sharpness.
+
+Required properties:
+- compatible: value should be one of the following
+               "rockchip,rk3228-rga";
+               "rockchip,rk3288-rga";
+               "rockchip,rk3399-rga";
+
+- interrupts: RGA interrupt number.
+
+- clocks: phandle to RGA sclk/hclk/aclk clocks
+
+- clock-names: should be "aclk" "hclk" and "sclk"
+
+- resets: Must contain an entry for each entry in reset-names.
+  See ../reset/reset.txt for details.
+- reset-names: should be "core" "axi" and "ahb"
+
+Example:
+SoC specific DT entry:
+       rga: rga@ff680000 {
+               compatible = "rockchip,rk3399-rga";
+               reg = <0xff680000 0x10000>;
+               interrupts = <GIC_SPI 55 IRQ_TYPE_LEVEL_HIGH>;
+               interrupt-names = "rga";
+               clocks = <&cru ACLK_RGA>, <&cru HCLK_RGA>, <&cru SCLK_RGA_CORE>;
+               clock-names = "aclk", "hclk", "sclk";
+
+               resets = <&cru SRST_RGA_CORE>, <&cru SRST_A_RGA>, <&cru SRST_H_RGA>;
+               reset-names = "core, "axi", "ahb";
+               status = "disabled";
+       };