ASoC: dt-bindings: add schema for WM8961

Create a simple DT schema for the existing Wolfson WM8961 driver so that
DT support can be added to the driver.

Signed-off-by: Doug Brown <doug@schmorgal.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221003032414.248326-2-doug@schmorgal.com


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