dt-bindings: drm/mediatek: Add Mediatek display subsystem dts binding

Add device tree binding documentation for the display subsystem in
Mediatek MT8173 SoCs.

Signed-off-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>