ASoC: dwc: Add documentation for I2S DT

DesignWare I2S controller

Required properties:

 - compatible : Must be "snps,designware-i2s"

 - reg : Must contain the I2S core's registers location and length

 - clocks : Pairs of phandle and specifier referencing the controller's

   clocks. The controller expects one clock: the clock used as the sampling

   rate reference clock sample.

 - clock-names : "i2sclk" for the sample rate reference clock.

 - dmas: Pairs of phandle and specifier for the DMA channels that are used by

   the core. The core expects one or two dma channels: one for transmit and

   one for receive.

 - dma-names : "tx" for the transmit channel, "rx" for the receive channel.

For more details on the 'dma', 'dma-names', 'clock' and 'clock-names'

properties please check:

	* resource-names.txt

	* clock/clock-bindings.txt

	* dma/dma.txt

Example:

	soc_i2s: i2s@7ff90000 {

		compatible = "snps,designware-i2s";

		reg = <0x0 0x7ff90000 0x0 0x1000>;

		clocks = <&scpi_i2sclk 0>;

		clock-names = "i2sclk";

		#sound-dai-cells = <0>;

		dmas = <&dma0 5>;

		dma-names = "tx";

	};