ASoC: tlv320aic31xx: configure output common-mode voltage

The tlv320aic31xx devices allow to adjust the output common-mode voltage
for best analog performance. The datasheet states that the common mode
voltage should be set to be <= AVDD/2.

This changes allows to configure the output common-mode voltage via a DT
property. If the property is absent the voltage is automatically chosen
as the highest voltage below/equal to AVDD/2.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Link: https://lore.kernel.org/r/20191118151207.28576-1-l.stach@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>