dt-bindings: display: meson-vpu: add third DPI output port

Add third port corresponding to the ENCL DPI encoder used to connect
to DSI or LVDS transceivers.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230512-amlogic-v6-4-upstream-dsi-ccf-vim3-v5-7-56eb7a4d5b8e@linaro.org