ASoC: dt-bindings: stm32: document audio of graph port for i2s

When linking the STM32 I2S 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/20230606115605.1633595-2-olivier.moysan@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>