ASoC: ads117x: Add bindings documentation for TI ADS117x ADC

Currently the binding is only made of the compatible string.

Signed-off-by: Florian Vaussard <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/sound/ti,ads117x.txt

···
+Texas Intstruments ADS117x ADC
+
+Required properties:
+
+  - compatible : "ti,ads1174" or "ti,ads1178"
+
+Example:
+
+ads1178 {
+	compatible = "ti,ads1178";
+};