ASoC: dt-bindings: add rockchip i2s bindings

* Rockchip I2S controller

The I2S bus (Inter-IC sound bus) is a serial link for digital

audio data transfer between devices in the system.

Required properties:

- compatible: should be one of the followings

   - "rockchip,rk3066-i2s": for rk3066

   - "rockchip,rk3188-i2s", "rockchip,rk3066-i2s": for rk3188

   - "rockchip,rk3288-i2s", "rockchip,rk3066-i2s": for rk3288

- reg: physical base address of the controller and length of memory mapped

  region.

- interrupts: should contain the I2S interrupt.

- #address-cells: should be 1.

- #size-cells: should be 0.

- dmas: DMA specifiers for tx and rx dma. See the DMA client binding,

	Documentation/devicetree/bindings/dma/dma.txt

- dma-names: should include "tx" and "rx".

- clocks: a list of phandle + clock-specifer pairs, one for each entry in clock-names.

- clock-names: should contain followings:

   - "i2s_hclk": clock for I2S BUS

   - "i2s_clk" : clock for I2S controller

Example for rk3288 I2S controller:

i2s@ff890000 {

	compatible = "rockchip,rk3288-i2s", "rockchip,rk3066-i2s";

	reg = <0xff890000 0x10000>;

	interrupts = <GIC_SPI 85 IRQ_TYPE_LEVEL_HIGH>;

	#address-cells = <1>;

	#size-cells = <0>;

	dmas = <&pdma1 0>, <&pdma1 1>;

	dma-names = "rx", "tx";

	clock-names = "i2s_hclk", "i2s_clk";

	clocks = <&cru HCLK_I2S0>, <&cru SCLK_I2S0>;

};