dt: wm8904: add device tree binding document

Signed-off-by: NBo Shen <voice.shen@atmel.com>
Signed-off-by: NMark Brown <broonie@linaro.org>

Documentation/devicetree/bindings/sound/wm8904.txt

+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";
+};