ASoC: audio-graph-card: update bindings for amplifier support

The audio-graph-card should be able to support widgets and routing in
the same way as what simple-audio-card does.  The patch adds the
properties into audio-graph-card bindings.  Then an optional property
'pa-gpios' for controlling external amplifier, which depends on DAPM
widgets and routing, is added.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/audio-graph-card.txt

@@ -10,6 +10,8 @@ see ${LINUX}/Documentation/devicetree/bindings/sound/simple-card.txt
 Below are same as Simple-Card.
 
 - label
+- widgets
+- routing
 - dai-format
 - frame-master
 - bitclock-master
@@ -24,6 +26,9 @@ Required properties:
 - compatible				: "audio-graph-card";
 - dais					: list of CPU DAI port{s}
 
+Optional properties:
+- pa-gpios: GPIO used to control external amplifier.
+
 Example: Single DAI case
 
 	sound_card {