samples/basic: disco: LED0 and LED1 might not share GPIO controller

In disco basic sample, same GPIO controller was used for LED0
and LED1, which is actually a particular case.

This change implements the general case, where LED0 an LED1 might
not have the same gpio controller.
Tested on nucleo_f429zi and frdm_k64f.

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