ASoC: dt-bindings: realtek,rt5640: Document port node

Various boards that use the rt5640 audio codec define a 'port' child
node under the codec node to describe the interface between it and the
SoC that it is connected to. The binding document for the rt5640 codec
does not define the 'port' child node and so this is generating warnings
when running the DTB checks for these boards. Add the 'port' node to the
binding document for the rt5640 codec to fix this.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/20260108143158.351223-4-jonathanh@nvidia.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/realtek,rt5640.yaml

@@ -130,6 +130,10 @@ properties:
       - 2 # Scale current by 1.0
       - 3 # Scale current by 1.5
 
+  port:
+    $ref: audio-graph-port.yaml#
+    unevaluatedProperties: false
+
 required:
   - compatible
   - reg