ASoC: dt-bindings: rockchip: i2s-tdm: Document audio graph port

Document the 'port' property to allow the Rockchip I2S TDM controller to
be used in conjunction with the audio-graph-card.

The property will be used to provide an endpoint for binding to the
other side of the audio link.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20230315114806.3819515-6-cristian.ciocaltea@collabora.com


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