drm/mediatek: Add ddp routing for mt7623

on BPi-R2/mt7623 main-path have to be routed to DPI0 (hdmi) instead of DSI0
using compatible "mt7623-mmsys" already defined in dts

Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>