ASoC: Add device tree documentation file for wm8782 stereo DAC

Add a device tree documentation file for the wm8782 stereo DAC to describe
the regulator handles.

Signed-off-by: Daniel Mack <[e-mail]>
Acked-by: Charles Keepax <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/sound/wm8782.txt

···
+WM8782 stereo ADC
+
+This device does not have any control interface or reset pins.
+
+Required properties:
+
+ - compatible  : "wlf,wm8782"
+ - Vdda-supply : phandle to a regulator for the analog power supply (2.7V - 5.5V)
+ - Vdd-supply  : phandle to a regulator for the digital power supply (2.7V - 3.6V)
+
+Example:
+
+wm8782: stereo-adc {
+	compatible = "wlf,wm8782";
+	Vdda-supply = <&vdda_supply>;
+	Vdd-supply = <&vdd_supply>;
+};