MAINTAINERS: add Mediatek SoC mailing list

Add the new list that Mediatek specific patches should also be
directed to.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Olof Johansson <olof@lixom.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>