ASoC: dt-bindings: linux,spdif: Add "port" node

The SPDIF receiver/transmitter can be connected to a DAI device via OF
graph port. It is already in use and supported.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20250910233615.775397-2-robh@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/linux,spdif.yaml

@@ -23,6 +23,9 @@ properties:
 
   sound-name-prefix: true
 
+  port:
+    $ref: /schemas/graph.yaml#/properties/port
+
 required:
   - "#sound-dai-cells"
   - compatible