ASoC: Add documentation for pcm3060 property out-single-ended

Output of pcm3060 codec may be configured as single-ended or differential
Signed-off-by: Kirill Marinushkin <kmarinushkin@birdec.tech>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/pcm3060.txt

@@ -9,9 +9,15 @@ Required properties:
 - reg : the I2C address of the device for I2C, the chip select
         number for SPI.
 
+Optional properties:
+
+- ti,out-single-ended: "true" if output is single-ended;
+                       "false" or not specified if output is differential.
+
 Examples:
 
 	pcm3060: pcm3060@46 {
 		 compatible = "ti,pcm3060";
 		 reg = <0x46>;
+		 ti,out-single-ended = "true";
 	};