ASoC: codecs/jz4760: add digital gain controls

Both the DAC and ADC have digital gain controls available
for their mixers, which go from -31 to 0db by step of 1db.

Signed-off-by: Christophe Branchereau <cbranchereau@gmail.com>
Link: https://lore.kernel.org/r/20230122210703.2552384-1-cbranchereau@gmail.com


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