ASoC: Documentation: Add google, cros-ec-codec

Add documentation for Chrome EC codec driver.
Signed-off-by: Cheng-Yi Chiang <cychiang@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/google,cros-ec-codec.txt

+* Audio codec controlled by ChromeOS EC
+
+Google's ChromeOS EC codec is a digital mic codec provided by the
+Embedded Controller (EC) and is controlled via a host-command interface.
+
+An EC codec node should only be found as a sub-node of the EC node (see
+Documentation/devicetree/bindings/mfd/cros-ec.txt).
+
+Required properties:
+- compatible: Must contain "google,cros-ec-codec"
+- #sound-dai-cells: Should be 1. The cell specifies number of DAIs.
+- max-dmic-gain: A number for maximum gain in dB on digital microphone.
+
+Example:
+
+cros-ec@0 {
+	compatible = "google,cros-ec-spi";
+
+	...
+
+	cros_ec_codec: ec-codec {
+		compatible = "google,cros-ec-codec";
+		#sound-dai-cells = <1>;
+		max-dmic-gain = <43>;
+	};
+};

MAINTAINERS

@@ -3687,6 +3687,13 @@ N:	cros_ec
 N:	cros-ec
 F:	drivers/power/supply/cros_usbpd-charger.c
 
+CHROMEOS EC CODEC DRIVER
+M:	Cheng-Yi Chiang <cychiang@chromium.org>
+S:	Maintained
+R:	Enric Balletbo i Serra <enric.balletbo@collabora.com>
+R:	Guenter Roeck <groeck@chromium.org>
+F:	Documentation/devicetree/bindings/sound/google,cros-ec-codec.txt
+
 CIRRUS LOGIC AUDIO CODEC DRIVERS
 M:	Brian Austin <brian.austin@cirrus.com>
 M:	Paul Handrigan <Paul.Handrigan@cirrus.com>