ASoC: audio-graph-card2-custom-sample.dtsi: add convert-rate for DPCM (MIXer)

audio-graph-card2-custom-sample.dtsi is assuming that
DPCM sample is MIXer connection.

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

CPU3/CPU4 need to convert rate in this case.
This patch adds missing "convert-rate" setting sample for it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/8735b38fta.wl-kuninori.morimoto.gx@renesas.com


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