ASoC: ads117x: Add bindings documentation for TI ADS117x ADC

Currently the binding is only made of the compatible string.
Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Signed-off-by: Mark Brown <broonie@kernel.org>

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

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