ASoC: Mediatek: Add document for mt8183-mt6358-ts3a227-max98357

This patch adds document for the machine board with
mt6358, ts3a227 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-mt6358-ts3a227-max98357.txt

+MT8183 with MT6358, TS3A227 and MAX98357 CODECS
+
+Required properties:
+- compatible : "mediatek,mt8183_mt6358_ts3a227_max98357"
+- mediatek,headset-codec: the phandles of ts3a227 codecs
+- mediatek,platform: the phandle of MT8183 ASoC platform
+
+Example:
+
+	sound {
+		compatible = "mediatek,mt8183_mt6358_ts3a227_max98357";
+		mediatek,headset-codec = <&ts3a227>;
+		mediatek,platform = <&afe>;
+	};
+