ASoC: tlv320aic31xx: Add Volume Soft Stepping control

Chip supports soft stepping of volume changes and it is enabled by
default.

This patch adds a control for it, so it could be either made slower
(two sample periods per step instead of one), or disabled.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Link: https://lore.kernel.org/r/20191129132719.11603-1-nikita.yoush@cogentembedded.com


Signed-off-by: Mark Brown <broonie@kernel.org>