MAINTAINERS: add myself as maintainer of fbdev

MAINTAINERS: add myself as maintainer of fbdev

I would like to help with fbdev maintenance.  I can dedicate some time
for reviewing and handling patches but won't have time for much more.

The subsystem will remain in maintenance mode (no new drivers will be
added to it).

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>