powerpc: remove redundant GENERIC_GPIO selection

ARCH_REQUIRE_GPIOLIB selects GENERIC_GPIO, so there is no need to select
it explicitly.

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