ASoC: dt-bindings: realtek,rt5616: Document audio graph port

The codec can be used in conjunction with an audio-graph-card to provide
an endpoint for binding with the other side of the audio link.

Document the 'port' property that is used for this to prevent
dtbscheck errors like:

rockchip/rk3588-nanopc-t6-lts.dtb: codec@1b: Unevaluated properties are not allowed ('port' was unexpected)
from schema $id: http://devicetree.org/schemas/sound/realtek,rt5616.yaml#

Signed-off-by: Heiko Stuebner <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/sound/realtek,rt5616.yaml

···
     items:
       - const: mclk
 
+  port:
+    $ref: audio-graph-port.yaml#
+    unevaluatedProperties: false
+
 required:
   - compatible
   - reg