dt-bindings: display: panel-simple-dsi: document port

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

  msm8916-samsung-a3u-eur.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: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20230326155425.91181-4-krzysztof.kozlowski@linaro.org
Signed-off-by: Rob Herring <robh@kernel.org>