ASoC: Add device tree binding for WM8731

Tested with the famous "hey, look! this compiles" test plan.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Acked by: Grant Likely <grant.likely@secretlab.ca>

Documentation/devicetree/bindings/sound/wm8731.txt

@@ -0,0 +1,18 @@
+WM8731 audio CODEC
+
+This device supports both I2C and SPI (configured with pin strapping
+on the board).
+
+Required properties:
+
+  - compatible : "wlf,wm8731"
+
+  - reg : the I2C address of the device for I2C, the chip select
+          number for SPI.
+
+Example:
+
+codec: wm8731@1a {
+	compatible = "wlf,wm8731";
+	reg = <0x1a>;
+};