ASoC: rsnd: enable single DVC valume settings

DVC controls some digital volume features.
Some of them requests values for "each channels",
but, some of them requests values for "feature".
And, Volume Ramp has "feature" settings.
This patch adds rsnd_dvc_cfg_s and care about
single settings.
Compiler will report like below at this point,
but, it will be removed if Volume Ramp was supported.
  warning: '_rsnd_dvc_pcm_new_s' defined but not used

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