dt: wm8904: add device tree binding document

WM8904 audio CODEC

This device supports I2C only.

Required properties:

  - compatible: "wlf,wm8904"

  - reg: the I2C address of the device.

  - clock-names: "mclk"

  - clocks: reference to

    <Documentation/devicetree/bindings/clock/clock-bindings.txt>

Pins on the device (for linking into audio routes):

  * IN1L

  * IN1R

  * IN2L

  * IN2R

  * IN3L

  * IN3R

  * HPOUTL

  * HPOUTR

  * LINEOUTL

  * LINEOUTR

  * MICBIAS

Examples:

codec: wm8904@1a {

	compatible = "wlf,wm8904";

	reg = <0x1a>;

	clocks = <&pck0>;

	clock-names = "mclk";

};