ASoC: rt5640: Allow configuration of LOUT to mono differential mode

LOUT output can produce either single-ended stereo signals, or mono
differential signal. Some applications are wired to use LOUT in mono
differential mode. Allow to configure it via device property.

Signed-off-by: Ondrej Jirman <megi@xff.cz>
Signed-off-by: Jarrah Gosbell <kernel@undef.tools>
Link: https://lore.kernel.org/r/20221209105621.39237-1-kernel@undef.tools
Signed-off-by: Mark Brown <broonie@kernel.org>