ASoC: adau1977: Add MICBIAS example in DT bindings

Add MICBIAS property to the optional devicetree bindings.
Signed-off-by: Bogdan Togorean <bogdan.togorean@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/adi,adau1977.txt

@@ -23,6 +23,12 @@ Optional properties:
  - DVDD-supply: supply voltage for the digital core, please consult
                 Documentation/devicetree/bindings/regulator/regulator.txt
 
+- adi,micbias: configures the voltage setting for the MICBIAS pin.
+		Select 0/1/2/3/4/5/6/7/8 to specify MICBIAS voltage
+		5V/5.5V/6V/6.5V/7V/7.5V/8V/8.5V/9V
+		If not specified the default value will be "7" meaning 8.5 Volts.
+		This property is only valid for the ADAU1977
+
 For required properties on SPI, please consult
 Documentation/devicetree/bindings/spi/spi-bus.txt
 
@@ -40,6 +46,7 @@ Examples:
 		AVDD-supply = <&regulator>;
 		DVDD-supply = <&regulator_digital>;
 
+		adi,micbias = <3>;
 		reset_gpio = <&gpio 10 GPIO_ACTIVE_LOW>;
 	};