dt-bindings: mediatek,dp: Add Display Port binding

This controller is present on several mediatek hardware. Currently
mt8195 and mt8395 have this controller without a functional difference,
so only one compatible field is added.

The controller can have two forms, as a normal display port and as an
embedded display port.

Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Signed-off-by: Guillaume Ranquet <granquet@baylibre.com>
Signed-off-by: Bo-Chen Chen <rex-bc.chen@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220901044149.16782-2-rex-bc.chen@mediatek.com