ASoC: binding: add widgets.txt

Suggested-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

Documentation/devicetree/bindings/sound/widgets.txt

+Widgets:
+
+This mainly specifies audio off-codec DAPM widgets.
+
+Each entry is a pair of strings in DT:
+
+	"template-wname", "user-supplied-wname"
+
+The "template-wname" being the template widget name and currently includes:
+"Microphone", "Line", "Headphone" and "Speaker".
+
+The "user-supplied-wname" being the user specified widget name.
+
+For instance:
+	simple-audio-widgets =
+		"Microphone", "Microphone Jack",
+		"Line", "Line In Jack",
+		"Line", "Line Out Jack",
+		"Headphone", "Headphone Jack",
+		"Speaker", "Speaker External";