selftests: gpio: add CONFIG_GPIO_CDEV to config

GPIO CDEV is now optional and required for the selftests so add it to
the config.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>