ASoC: atmel-i2s: dt-bindings: add DT bindings for I2S controller

* Atmel I2S controller

Required properties:

- compatible:     Should be "atmel,sama5d2-i2s".

- reg:            Should be the physical base address of the controller and the

                  length of memory mapped region.

- interrupts:     Should contain the interrupt for the controller.

- dmas:           Should be one per channel name listed in the dma-names property,

                  as described in atmel-dma.txt and dma.txt files.

- dma-names:      Two dmas have to be defined, "tx" and "rx".

                  This IP also supports one shared channel for both rx and tx;

                  if this mode is used, one "rx-tx" name must be used.

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

                  Please refer to clock-bindings.txt.

- clock-names:    Should be one of each entry matching the clocks phandles list:

                  - "pclk" (peripheral clock) Required.

                  - "gclk" (generated clock) Optional (1).

                  - "aclk" (Audio PLL clock) Optional (1).

                  - "muxclk" (I2S mux clock) Optional (1).

Optional properties:

- pinctrl-0:      Should specify pin control groups used for this controller.

- princtrl-names: Should contain only one value - "default".

(1) : Only the peripheral clock is required. The generated clock, the Audio

      PLL clock adn the I2S mux clock are optional and should only be set

      together, when Master Mode is required.

Example:

	i2s@f8050000 {

		compatible = "atmel,sama5d2-i2s";

		reg = <0xf8050000 0x300>;

		interrupts = <54 IRQ_TYPE_LEVEL_HIGH 7>;

		dmas = <&dma0

			(AT91_XDMAC_DT_MEM_IF(0) | AT91_XDMAC_DT_PER_IF(1) |

			 AT91_XDMAC_DT_PERID(31))>,

		       <&dma0

			(AT91_XDMAC_DT_MEM_IF(0) | AT91_XDMAC_DT_PER_IF(1) |

			 AT91_XDMAC_DT_PERID(32))>;

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

		clocks = <&i2s0_clk>, <&i2s0_gclk>, <&audio_pll_pmc>, <&i2s0muxck>;

		clock-names = "pclk", "gclk", "aclk", "muxclk";

		pinctrl-names = "default";

		pinctrl-0 = <&pinctrl_i2s0_default>;

	};