device-tree bindings for rockchip 2D raster graphic acceleration controller (RGA)

RGA is a standalone 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,rga2";
		"rockchip,rk312x-rga";

- interrupts: RGA interrupt specifier.

- clocks: phandle to RGA sclk/hclk/aclk clocks

- clock-names: should be "aclk", "hclk" and "sclk"

- status: the dsi host status;
	<disabled>: open the dsi host;
	<okay>:close the dsi host;

Optional Properties:
- dma-coherent: When import dma_buf to rga driver, dma_map_sg will always do cpu cache sync, it cause low performance.Actually we don't want to do cpu cache sync on this context,So set rga device with dma-coherent to skip cpu cache sync.

Example:
SoC-specific DT entry:
	rga: rga@ff680000 {
		compatible = "rockchip,rga2";
		reg = <0xff680000 0x10000>;
		interrupts = <GIC_SPI 55 IRQ_TYPE_LEVEL_HIGH>;
		clocks = <&cru ACLK_RGA>, <&cru HCLK_RGA>, <&cru SCLK_RGA_CORE>;
		clock-names = "aclk", "hclk", "sclk";
		dma-coherent;
		status = "disabled";
	};