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

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 <krzysztof.kozlowski@linaro.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20230326204224.80181-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring <robh@kernel.org>