ASoC: audio-graph-scu-card: support 2nd codec endpoint on DT

audio-graph-scu-card can handle below connection which is mainly
for sound mixing purpose.

	+----------+   +-------+
	| CPU0--+--|-->| Codec |
	|       |  |   +-------+
	| CPU1--+  |
	+----------+

>From OF-graph point of view, it should have
CPU0 <-> Codec, and CPU1 <-> Codec on DT.
But current driver doesn't care about 2nd connection
of Codec, because it is dummy from DPCM point of view.

This patch can care 2nd Codec connection, and it should be
supported from OF-graph point of view.
It still have backward compatibility.

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