ASoC: pcm3168a: Document optional RST gpio

On boards where the RST line is not pulled up, but it is connected to a
GPIO line this property must present in order to be able to enable the
codec.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20191113124734.27984-2-peter.ujfalusi@ti.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/ti,pcm3168a.txt

@@ -25,6 +25,12 @@ Required properties:
 
 For required properties on SPI/I2C, consult SPI/I2C device tree documentation
 
+Optional properties:
+
+  - rst-gpios : Optional RST gpio line for the codec
+		RST = low: device power-down
+		RST = high: device is enabled
+
 Examples:
 
 i2c0: i2c0@0 {
@@ -34,6 +40,7 @@ i2c0: i2c0@0 {
 	pcm3168a: audio-codec@44 {
 		compatible = "ti,pcm3168a";
 		reg = <0x44>;
+		rst-gpios = <&gpio0 4 GPIO_ACTIVE_HIGH>;
 		clocks = <&clk_core CLK_AUDIO>;
 		clock-names = "scki";
 		VDD1-supply = <&supply3v3>;