ASoC: dt-bindings: renesas,rz-ssi: Document port property

Document optional port property that connects to I2S signals.
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://patch.msgid.link/20240725084559.13127-2-biju.das.jz@bp.renesas.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/renesas,rz-ssi.yaml

@@ -87,6 +87,10 @@ properties:
   '#sound-dai-cells':
     const: 0
 
+  port:
+    $ref: audio-graph-port.yaml#/definitions/port-base
+    description: Connection to controller providing I2S signals
+
 required:
   - compatible
   - reg