ASoC: rsnd: enable enumerated DVC valume settings

DVC controls some digital volume features.
Volume Ramp is listed as "XX dB / YY steps",
and this enumerated settings are easy for users.
This patch adds rsnd_dvc_cfg_e and care about
enumerated settings.
Compiler will report like below at this point,
but, it will be removed if Volume Ramp was supported.
  warning: '_rsnd_dvc_pcm_new_e' defined but not used

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