ASoC: dt-bindings: rockchip,rk3308-codec: add port property

Fix DTB warnings when rk3308-codec used with audio-graph-card by
documenting port property:

codec@ff560000: 'port' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Dmitry Yashin <[e-mail]>
Reviewed-by: Luca Ceresoli <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/sound/rockchip,rk3308-codec.yaml

···
       - const: mclk_rx
       - const: hclk
 
+  port:
+    $ref: audio-graph-port.yaml#
+    unevaluatedProperties: false
+
   resets:
     maxItems: 1