ASoC: dt-bindings: ep93xx: Document Audio Port support

Document Audio Graph Port support in binding document.

Signed-off-by: Nikita Shubin <[e-mail]>
Tested-by: Alexander Sverdlin <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Reviewed-by: Guenter Roeck <[e-mail]>
Reviewed-by: Mark Brown <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Acked-by: Miquel Raynal <[e-mail]>
Acked-by: Vinod Koul <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>

Documentation/devicetree/bindings/sound/cirrus,ep9301-i2s.yaml

···
       - const: tx
       - const: rx
 
+  port:
+    $ref: audio-graph-port.yaml#
+    unevaluatedProperties: false
+
 required:
   - compatible
   - '#sound-dai-cells'