MAINTAINERS: change mediatek wiki page

The old wiki page unfortunately got lost by server crash.
The new wiki can be found on the kernel.org infrastructure

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Link: https://lore.kernel.org/r/20201020152639.21950-1-matthias.bgg@kernel.org


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>