avr32: default GENERIC_GPIO to false

This will help removing the GENERIC_GPIO option.

GPIO access is done through gpiolib, which selects GENERIC_GPIO anyway.

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