samples/drivers: gpio: LED0 and SW0 might not share GPIO controller

In gpio driver sample, same GPIO controller was used for out pin
(LED0) and in pin (SW0), which is actually a particular case.

This change implements the general case, where button an led might
not have the same gpio controller.
Tested on nucleo_f429zi and frdm_k64f.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>