Remove GENERIC_GPIO config option

GENERIC_GPIO has been made equivalent to GPIOLIB in architecture code
and all driver code has been switch to depend on GPIOLIB. It is thus
safe to have GENERIC_GPIO removed.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>