dt-bindings: sound: add dmicen property in dmic driver

there may use enable pin to control dmic start and stop,
so add this property in dt-bindings.

Signed-off-by: Lin Huang <hl@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/dmic.txt

@@ -5,8 +5,12 @@ This device support generic PDM digital microphone.
 Required properties:
 	- compatible: should be "dmic-codec".
 
+Optional properties:
+	- dmicen-gpios: GPIO specifier for dmic to control start and stop
+
 Example node:
 
 	dmic_codec: dmic@0 {
 		compatible = "dmic-codec";
+		dmicen-gpios = <&gpio4 3 GPIO_ACTIVE_HIGH>;
 	};