ASoC: audio-graph-card: merge audio-graph-scu-card on Doc

audio-graph-card and audio-graph-scu-card are very similar driver,
but the former is supporting normal sound card,
the latter is supporting DPCM sound card.

We couldn't use normal sound and DPCM sound in same sound card by
audio-graph-card.
This patch merges both feature into it on Documentation.
Now we can use both feature on same driver.

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