ASoC: cs4341: Add DT bindings documentation for CS4341 DAC

Cirrus Logic CS4341 audio DAC

This device supports both I2C and SPI (configured with pin strapping

on the board).

Required properties:

  - compatible: "cirrus,cs4341a"

  - reg : the I2C address of the device for I2C, the chip select

          number for SPI.

For required properties on I2C-bus, please consult

Documentation/devicetree/bindings/i2c/i2c.txt

For required properties on SPI-bus, please consult

Documentation/devicetree/bindings/spi/spi-bus.txt

Example:

	codec: cs4341@0 {

		#sound-dai-cells = <0>;

		compatible = "cirrus,cs4341a";

		reg = <0>;

		spi-max-frequency = <6000000>;

	};