ASoC: dt-bindings: mt8183-da7219: add a property "mediatek, hdmi-codec"

Adds an optional property "mediatek,hdmi-codec".
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20200710052505.3664118-5-tzungbi@google.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/mt8183-da7219-max98357.txt

@@ -6,11 +6,15 @@ Required properties:
 - mediatek,headset-codec: the phandles of da7219 codecs
 - mediatek,platform: the phandle of MT8183 ASoC platform
 
+Optional properties:
+- mediatek,hdmi-codec: the phandles of HDMI codec
+
 Example:
 
 	sound {
 		compatible = "mediatek,mt8183_da7219_max98357";
 		mediatek,headset-codec = <&da7219>;
+		mediatek,hdmi-codec = <&it6505dptx>;
 		mediatek,platform = <&afe>;
 	};