device-tree bindings for rockchip VPU codec

Rockchip (Video Processing Unit) present in various Rockchip platforms,
such as RK3288 and RK3399.

Required properties:
- compatible: value should be one of the following
		"rockchip,rk3288-vpu";
		"rockchip,rk3399-vpu";
- interrupts: encoding and decoding interrupt specifiers
- interrupt-names: should be "vepu" and "vdpu"
- clocks: phandle to VPU aclk, hclk clocks
- clock-names: should be "aclk" and "hclk"
- power-domains: phandle to power domain node
- iommus: phandle to a iommu node

Example:
SoC-specific DT entry:
	vpu: video-codec@ff9a0000 {
		compatible = "rockchip,rk3288-vpu";
		reg = <0x0 0xff9a0000 0x0 0x800>;
		interrupts = <GIC_SPI 9 IRQ_TYPE_LEVEL_HIGH>,
			     <GIC_SPI 10 IRQ_TYPE_LEVEL_HIGH>;
		interrupt-names = "vepu", "vdpu";
		clocks = <&cru ACLK_VCODEC>, <&cru HCLK_VCODEC>;
		clock-names = "aclk", "hclk";
		power-domains = <&power RK3288_PD_VIDEO>;
		iommus = <&vpu_mmu>;
	};