blackfin: force use of gpiolib

Set the GENERIC_GPIO option to false by default and force the use of
gpiolib instead of making it optional, to prepare for the removal of
GENERIC_GPIO.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Steven Miao <realmz6@gmail.com>