ASoC: dt-bindings: document audio of graph port for cs42l51

When linking the CS42L51 to another DAI component, according
to audio graph cards bindings, an OF graph port property is expected
in the node. Document the port property.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230606115605.1633595-3-olivier.moysan@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>