dt-bindings: display: mediatek: convert the dpi bindings to yaml

Convert display/mediatek/mediatek,dpi.txt to
display/mediatek/mediatek,dpi.yaml and remove the old text
bindings.

Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>