ASoC: codecs/jz4770: Add missing gain control after DAC/ADC mixer

The capture and playback paths both have a configurable gain after their
respective mixer, which can be set from -31 dB to 0 dB in 32 steps.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20211125232543.117074-1-paul@crapouillou.net


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