ASoC: audio-graph-card: add widgets and routing for external amplifier support

It's very common that audio card has a machine level amplifier which is
controlled by GPIO.  The patch adds DAPM widgets and routing support
into audio-graph-card driver, and creates an output driver widget with
event to control the amplifier via GPIO.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>