openrisc: default GENERIC_GPIO to false

This is one step towards the removal of the GENERIC_GPIO option.
OpenRISC mandates the use of GPIOLIB, which enables GENERIC_GPIO anyway,
so this patch should be a no-op.

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