arm: remove redundant GENERIC_GPIO selection

GENERIC_GPIO was selected next to ARCH_REQUIRE_GPIOLIB, which itself
selects 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>