drm/mediatek: update dt-bindings for mt2712

Update device tree binding documentation for the display subsystem for
Mediatek MT2712 SoCs.

Signed-off-by: Stu Hsieh <stu.hsieh@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>