ASoC: rsnd: add rsnd_scu_transfer_start()

Renesas sound has SRC (= Sampling Rate Converter),
but, the HW implementation depends on its generation.
It was part of SRU on Gen1, and SCU on Gen2.
This SCU needs DMA transfer to use it.
Current rsnd driver is using it as DMA transfer buffer
(= no rate convert), and Gen1 is only supported at this point.

This patch cleanup it with focusing about SRC and Gen2 part.

SRC_CTRL/BUSIF_MODE are used for transfer start.
This patch adds rsnd_scu_transfer_start() and merge these

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