MAINTAINERS: Add maintainers entry for Mediatek SoCs

I plan to stay with the Mediatek SoCs for the next future
and hope to expand its support along the way with the help
of a whole bunch of people.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>