ASoC: Mediatek: Add document for mt8183-da7219-max98357

This patch adds document for the machine board with
mt6358, da7219 and max98357 codecs.
Signed-off-by: Shunli Wang <shunli.wang@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

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

+MT8183 with MT6358, DA7219 and MAX98357 CODECS
+
+Required properties:
+- compatible : "mediatek,mt8183_da7219_max98357"
+- mediatek,headset-codec: the phandles of da7219 codecs
+- mediatek,platform: the phandle of MT8183 ASoC platform
+
+Example:
+
+	sound {
+		compatible = "mediatek,mt8183_da7219_max98357";
+		mediatek,headset-codec = <&da7219>;
+		mediatek,platform = <&afe>;
+	};
+