ASoC: st,stm32-sai: Document audio OF graph port

It is expected that the SAI subnodes would contain audio OF graph port
with endpoint to link it with the other side of audio link. Document
the port: property.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Olivier Moysan <olivier.moysan@foss.st.com>
Link: https://lore.kernel.org/r/20220927002004.685108-1-marex@denx.de


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