ASoC: rsnd: add rsnd_dvc_initialize_lock/unlock()

Renesas SCU (Sampling Rate Convert Unit) includes SRC/CTU/MIX/DVC,
and these have similar register. xxxIR (Initialization Register) is one
of them. These xxxIR need be set to 1 during initialization.
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_dvc_initialize_lock/unlock() 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>