ASoC: rt5640: Allow to describe how LOUT is wired

Depending on HW design, som boards may expect mono differential output
on LOUT. Describe a property that enables it.

Signed-off-by: Ondrej Jirman <megi@xff.cz>
Signed-off-by: Jarrah Gosbell <kernel@undef.tools>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221209105621.39237-2-kernel@undef.tools
Signed-off-by: Mark Brown <broonie@kernel.org>