ASoC: pcm186x: Add PCM186x binding documentation

Add the dt-binding documentation for the TI PCM186x 2ch and 4ch Audio
ADCs With Universal Front End.
Signed-off-by: Andrew F. Davis <afd@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/pcm186x.txt

+Texas Instruments PCM186x Universal Audio ADC
+
+These devices support both I2C and SPI (configured with pin strapping
+on the board).
+
+Required properties:
+
+ - compatible : "ti,pcm1862",
+                "ti,pcm1863",
+                "ti,pcm1864",
+                "ti,pcm1865"
+
+ - reg : The I2C address of the device for I2C, the chip select
+         number for SPI.
+
+ - avdd-supply: Analog core power supply (3.3v)
+ - dvdd-supply: Digital core power supply
+ - iovdd-supply: Digital IO power supply
+        See regulator/regulator.txt for more information
+
+CODEC input pins:
+ * VINL1
+ * VINR1
+ * VINL2
+ * VINR2
+ * VINL3
+ * VINR3
+ * VINL4
+ * VINR4
+
+The pins can be used in referring sound node's audio-routing property.
+
+Example:
+
+	pcm186x: audio-codec@4a {
+		compatible = "ti,pcm1865";
+		reg = <0x4a>;
+
+		avdd-supply = <&reg_3v3_analog>;
+		dvdd-supply = <&reg_3v3>;
+		iovdd-supply = <&reg_1v8>;
+	};