ASoC: rsnd: add dma.c for Audio DMAC / Audio DMAC peri peri

Renesas sound driver had been assumed that Audio DMAC /
Audio DMAC peri peri are implemented by DMAEngine. But, result of DMA
ML discussion, it was concluded that it is not a general purpose DMAC.
And it should be moved from current DMAEngine to rsnd driver.
So, Audio DMAC peri peri become non DMAEngine.

OTOH, ALSA SoC has soc-generic-dmaengine-pcm implementation. but it
seems difficult to use this generic implementation on rsnd driver at
this point, since it needs to fallback to PIO mode if DMA can't use.
and additionally it needs 2 DMAC (= Audio DMAC / Audio DMAC peri peri).
These are not "generic" feature.

Of course I will try to use this generic dmaengine in the future somehow,
but just use current style at this point until it can formally use 2 DMACs.
This patch adds new dma.c and moves current dma code to dma.c from core.c.

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