xtensa: remove explicit selection of GENERIC_GPIO

The Xtensa GPIO driver uses gpiolib and cannot compile with only
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>