ASoC: dt-bindings: Add WM8978 Binding

Even though we had the wm8978 driver for some time and a number of
boards using it already, we never had a binding for it. Let's add it
based on what the driver expects and the boards are providing.

Cc: alsa-devel@alsa-project.org
Cc: devicetree@vger.kernel.org
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: patches@opensource.cirrus.com
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20210901091852.479202-2-maxime@cerno.tech


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