ASoC: audio-graph-card2-custom-sample.dtsi: add DPCM sample (Single)

This patch adds DPCM link Single-CPU/Codec sample to
audio-graph-card2-custom-sample.dtsi.
This sample is assuming MIXer connection.

	FE		BE
		****
	CPU3 -- *  * -- Codec3
	CPU4 -- *  *
		****

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87r1cqlu2w.wl-kuninori.morimoto.gx@renesas.com


Signed-off-by: Mark Brown <broonie@kernel.org>