dt-bindings: display: mediatek: add property to control mipi tx drive current

Add a property to control mipi tx drive current:
"drive-strength-microamp"

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>