ASoC: dt-bindings: realtek,rt5640: Document mclk

Commit eba5a0ba ("ASoC: dt-bindings: realtek,rt5640: Convert to
dtschema") converted the rt5640 dt-binding to yaml format but in the
process dropped 'clock' and 'clock-names' properties that are used to
specify the codec 'mclk'. This is causing DTB build warnings for boards
that use this codec and define an 'mclk' in device-tree. Update the
rt5640 binding document to add the optional mclk.

Fixes: eba5a0ba ("ASoC: dt-bindings: realtek,rt5640: Convert to dtschema")
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260108143158.351223-2-jonathanh@nvidia.com


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