mips: txx9: change GENERIC_GPIO to GPIOLIB

The GPIO driver for txx9 requires gpiolib, so this is more accurate and
let us get rid of one mention of GENERIC_GPIO which is due to disappear.

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