dt-bindings: display: mediatek: dpi: Revise mediatek strings to correct format

Strings replacement:
- s/mediatek/MediaTek/ in title.
- s/Mediatek/MediaTek/ in description.

Signed-off-by: Bo-Chen Chen <rex-bc.chen@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20220701035845.16458-3-rex-bc.chen@mediatek.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>