ASoC: google: dt-bindings: Add sc7180-trogdor machine bindings

Add devicetree bindings documentation file for sc7180 sound card.

Signed-off-by: Cheng-Yi Chiang <cychiang@chromium.org>
Reviewed-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20201027032234.1705835-2-cychiang@chromium.org
Signed-off-by: Mark Brown <broonie@kernel.org>