ASoC: dt-bindings: mt8183: add ec-codec

Add an optional property "ec-codec".  If specified, mt8183 could use the
"wake on voice" feature offered by EC codec.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20191019143504.3.Iec97a3f137148cdf316056612590b3e0b302f5f3@changeidSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/mt8183-mt6358-ts3a227-max98357.txt

@@ -6,12 +6,15 @@ Required properties:
 
 Optional properties:
 - mediatek,headset-codec: the phandles of ts3a227 codecs
+- mediatek,ec-codec: the phandle of EC codecs.
+                     See google,cros-ec-codec.txt for more details.
 
 Example:
 
 	sound {
 		compatible = "mediatek,mt8183_mt6358_ts3a227_max98357";
 		mediatek,headset-codec = <&ts3a227>;
+		mediatek,ec-codec = <&ec_codec>;
 		mediatek,platform = <&afe>;
 	};