ASoC: dt-bindings: wcd934x: add bindings for Headset Button detection

ASoC: dt-bindings: wcd934x: add bindings for Headset Button detection

Add bindings required for Multi Button Headset detection.
WCD934x support Headsets with upto 8 buttons including, impedance measurement
on both L/R Headset speakers and cross connection detection.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210604115230.23259-2-srinivas.kandagatla@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>