serial: remove blackfin drivers

The blackfin architecture is getting removed, so both the bfin_uart
and bfin_sport_uart can be removed as well.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Aaron Wu <aaron.wu@analog.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>