ASoC: rsnd: add MIX Volume Ramp support

Both DVC/MIX have Volume Ramp Control. This patch supprts MIX
Volume Ramp. One note is that main purpose of MIX Volume Ramp
is to reduce noise, thus, MIX Ramp range is very few if you
compare to DVC Volume Ramp (DVC = 5bit, MIX = 4bit).

You can use MIX Volume Ranp like below
	amixer set "MIX Ramp Up Rate"   "0.125 dB/1 step"
	amixer set "MIX Ramp Down Rate" "0.125 dB/1 step"
	amixer set "MIX Ramp" on
	aplay xxx.wav &
	amixer set "MIX",0  80%  // DAI0 Volume Down
	amixer set "MIX",1 100%  // DAI1 Volume Up

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>