ASoC: dt-bindings: cirrus,cs4271: Document mclk clock

ASoC: dt-bindings: cirrus,cs4271: Document mclk clock

The Cirrus CS4271 codec can use an external clock as an input Master
Clock. When no crystal is used, the CS4271 component considers its MCLK
pin as an input pin and expects the external clock connected to provide
the Master Clock.

This clock is not documented in the binding.

Add the missing clock.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20251029093921.624088-4-herve.codina@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>