dt-bindings: arm: mediatek: mmsys: Add OF graph support for board path

Document OF graph on MMSYS/VDOSYS: this supports up to three DDP paths
per HW instance (so potentially up to six displays for multi-vdo SoCs).

The MMSYS or VDOSYS is always the first component in the DDP pipeline,
so it only supports an output port with multiple endpoints - where each
endpoint defines the starting point for one of the (currently three)
possible hardware paths.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Tested-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Tested-by: Michael Walle <mwalle@kernel.org> # on kontron-sbc-i1200
Link: https://lore.kernel.org/r/20241017103809.156056-3-angelogioacchino.delregno@collabora.com


Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>