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

Documentation/devicetree/bindings/sound/rt5640.txt

@@ -20,6 +20,9 @@ Optional properties:
 - realtek,in3-differential
   Boolean. Indicate MIC1/2/3 input are differential, rather than single-ended.
 
+- realtek,lout-differential
+  Boolean. Indicate LOUT output is differential, rather than stereo.
+
 - realtek,ldo1-en-gpios : The GPIO that controls the CODEC's LDO1_EN pin.
 
 - realtek,dmic1-data-pin