ASoC: rsnd: Add Volume Ramp support

This patch adds Volume Ramp to Renesas sound driver.

amixer set "DVC Out" 100%
amixer set "DVC Out Ramp Up Rate"   "0.125 dB/64 steps"
amixer set "DVC Out Ramp Down Rate" "0.125 dB/512 steps"
amixer set "DVC Out Ramp" on
aplay xxx.wav &
amixer set "DVC Out"  80%  // Volume Down
amixer set "DVC Out" 100%  // Volume Up

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