ASoC: rsnd: src: make sure SRC soft reset

Renesas SCU (Sampling Rate Convert Unit) includes SRC/CTU/MIX/DVC,
and these have similar register. xxxRSR (Software reset Register) is one
of them. These xxxRSR need be set to 1 to 0 when software reset.
Current rsnd driver has src.c / dvc.c, and we will have mix.c.
It is readable if these have same named function.
This patch adds rsnd_src_soft_reset() and make sure it

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Keita Kobayashi <keita.kobayashi.ym@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>