[media] dt-bindings: Add a binding for Mediatek MDP

Add a DT binding documentation of MDP for the MT8173 SoC
from Mediatek

Signed-off-by: Minghsiu Tsai <minghsiu.tsai@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>