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

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

The Rockchip I2S controller is currently used in conjunction with
audio-graph-card to provide an endpoint for binding with the other side
of the audio link.

This is achieved via the 'port' property, which is not allowed:

  rk3399-rockpro64.dtb: i2s@ff890000: Unevaluated properties are not allowed ('port' was unexpected)

Fix the issue by documenting the missing property.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20230315114806.3819515-5-cristian.ciocaltea@collabora.com
Signed-off-by: Mark Brown <broonie@kernel.org>