ASoC: cs42l42: Add control for audio slow-start switch

This adds an ALSA control so that the slow-start audio ramp feature
can be disabled. This is useful for high-definition audio applications.

The register field is unusual in that it is a 3-bit field with only
two valid values, 000=off and 111=on.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20211101101006.13092-1-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>