mips: loongson: use GPIO driver on CONFIG_GPIOLIB

The GPIO driver uses gpiolib, thus it should be compiled when
CONFIG_GPIOLIB is defined and not only CONFIG_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>