mips: pnx833x: remove requirement for GENERIC_GPIO

pnx833x does not seem to use the generic gpio API.

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