spi/rockchip: add rockchip spi DT binding

* Rockchip SPI Controller

The Rockchip SPI controller is used to interface with various devices such as flash

and display controllers using the SPI communication interface.

Required Properties:

- compatible: should be one of the following.

    "rockchip,rk3066-spi" for rk3066.

    "rockchip,rk3188-spi", "rockchip,rk3066-spi" for rk3188.

    "rockchip,rk3288-spi", "rockchip,rk3066-spi" for rk3288.

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

       region.

- interrupts: The interrupt number to the cpu. The interrupt specifier format

              depends on the interrupt controller.

- clocks: Must contain an entry for each entry in clock-names.

- clock-names: Shall be "spiclk" for the transfer-clock, and "apb_pclk" for

			   the peripheral clock.

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

		Documentation/devicetree/bindings/dma/dma.txt

- dma-names: DMA request names should include "tx" and "rx" if present.

- #address-cells: should be 1.

- #size-cells: should be 0.

Example:

	spi0: spi@ff110000 {

		compatible = "rockchip,rk3066-spi";

		reg = <0xff110000 0x1000>;

		dmas = <&pdma1 11>, <&pdma1 12>;

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

		#address-cells = <1>;

		#size-cells = <0>;

		interrupts = <GIC_SPI 44 IRQ_TYPE_LEVEL_HIGH>;

		clocks = <&cru SCLK_SPI0>, <&cru PCLK_SPI0>;

		clock-names = "spiclk", "apb_pclk";

	};