ASoC: fsl: imx-audmix: cleanup platform which is using Generic DMA

If CPU is using soc-generic-dmaengine-pcm, Platform Component will be
same as CPU Component. In this case, we can use CPU dlc for Platform dlc.
This patch shares CPU dlc with Platform, and add comment.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Shengjiu Wang <shengjiu.wang@gmail.com>
Tested-by: Shengjiu Wang <shengjiu.wang@gmail.com>
Link: https://lore.kernel.org/r/878remf61j.wl-kuninori.morimoto.gx@renesas.com


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