fbdev: remove blackfin drivers

The blackfin architecture is getting removed, this removes the
associated fbdev drivers as well.

Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Aaron Wu <aaron.wu@analog.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>