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>