gpio: Further document optional GPIOLIB

Optional GPIOLIB as in not depended on or selected by a driver
should NOT use any *_optional() calls, this becomes paradoxical.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>