fbdev: Fix tmiofb driver dependencies

The tmiofb driver should not depend on MFD_CORE but on MFD_TMIO.
Without the tmio_core driver, tmiofb has no platform device to bind
to and is thus useless.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>