ASoC: rt5645: add DAC1 soft volume func control

This patch add an alsa control for DAC1 digital volume control function
selection. The options are:
0: Gain update immediately
1: Gain update when a zero crossing
2: Gain update when a zero crossing with a soft ramp

Signed-off-by: Bard Liao <bardliao@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>