gpio: Cleanup Kconfig for K64F

This patch addresses the following issues:
* Aligns the Kconfig code style with Zephyr projects requirements.
* Removes redundant "depends on" from Kconfig.

Change-Id: I4c8df0999f92a834d4023ce5856a2a6c39797c00
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@linaro.org>