ASoC: sunxi: DMIC: Add controls for adjusting the mic gains

ASoC: sunxi: DMIC: Add controls for adjusting the mic gains

The AllWinner H6 and later SoCs that sport a DMIC block contain a set of registers to control
the gain (left + right) of each of the four supported channels.

Add ASoC controls for changing each of the stereo channel gains using alsamixer and alike

Signed-off-by: Joao Schim <joao@schimsalabim.eu>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20240429194920.1596257-1-joao@schimsalabim.eu
Signed-off-by: Mark Brown <broonie@kernel.org>