ASoC: dt-bindings: everest,es8316: Document audio graph port

The ES8316 codec 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: codec@11: 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/20230315114806.3819515-4-cristian.ciocaltea@collabora.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/everest,es8316.yaml

@@ -28,6 +28,10 @@ properties:
     items:
       - const: mclk
 
+  port:
+    $ref: audio-graph-port.yaml#
+    unevaluatedProperties: false
+
   "#sound-dai-cells":
     const: 0