ASoC: add audio-graph-card support

OF-graph base DT binding are used on V4L2, and ALSA SoC is using
different style of DT today. Now ALSA SoC supports simple-card driver
for generic/simple sound card.
In the future, V4L2 / ALSA will support HDMI, and then, DT bindings
between V4L2 / ALSA should be merged.
This patch adds new Audio Graph Card which is OF-graph base of
simple-card

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>