unicore32: default GENERIC_GPIO to false

GPIO implementation for unicore32 uses gpiolib, which sets GENERIC_GPIO
when selected.

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