ASoC: mediatek: add bindings for wm8960 codec machine driver

add device tree bindings for mt2701-wm8960.
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/mt2701-wm8960.txt

+MT2701 with WM8960 CODEC
+
+Required properties:
+- compatible: "mediatek,mt2701-wm8960-machine"
+- mediatek,platform: the phandle of MT2701 ASoC platform
+- audio-routing: a list of the connections between audio
+- mediatek,audio-codec: the phandles of wm8960 codec
+- pinctrl-names: Should contain only one value - "default"
+- pinctrl-0: Should specify pin control groups used for this controller.
+
+Example:
+
+	sound:sound {
+		compatible = "mediatek,mt2701-wm8960-machine";
+		mediatek,platform = <&afe>;
+		audio-routing =
+			"Headphone", "HP_L",
+			"Headphone", "HP_R",
+			"LINPUT1", "AMIC",
+			"RINPUT1", "AMIC";
+		mediatek,audio-codec = <&wm8960>;
+		pinctrl-names = "default";
+		pinctrl-0 = <&aud_pins_default>;
+	};