ASoC: adau1761: Add PGA Slew time control

ASoC: adau1761: Add PGA Slew time control

The PGA Slew Time control allows to configure the rate with which the PGA
gain control ramps up/down to the target setting.

The PGA slew control is done via the ALC Control 0 register. There are 2
bits on that reg, that control PGA slew time, while the other bits control
parts of the ALC (automatic level control) block.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20190926115012.24049-1-alexandru.ardelean@analog.com
Signed-off-by: Mark Brown <broonie@kernel.org>