MAINTAINERS: change framebuffer maintainer

As Paul has not much time for it I take over maintaining the
framebuffer subsystem.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>