dt-bindings: display: xinpeng,xpp055c272: document port

Panels are supposed to have one port (defined in panel-common.yaml
binding):

px30-evb.dtb: panel@0: 'port' does not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Jagan Teki <[e-mail]>
Reviewed-by: Neil Armstrong <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/display/panel/xinpeng,xpp055c272.yaml

···
     const: xinpeng,xpp055c272
   reg: true
   backlight: true
+  port: true
   reset-gpios: true
   iovcc-supply:
     description: regulator that supplies the iovcc voltage
···
   - compatible
   - reg
   - backlight
+  - port
   - iovcc-supply
   - vci-supply
 
···
             backlight = <&backlight>;
             iovcc-supply = <&vcc_1v8>;
             vci-supply = <&vcc3v3_lcd>;
+
+            port {
+                mipi_in_panel: endpoint {
+                    remote-endpoint = <&mipi_out_panel>;
+                };
+            };
         };
     };