gpiolib: Warn on drivers still using static gpiobase allocation

In the preparation of getting completely rid of static gpiobase
allocation in the future, emit a warning in drivers still doing so.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>