drivers/wifi: Generalize GPIO configuration for the WINC1500 driver

Previous way was cumbersome (based on an old way for cc2520 actually),
so moving towards the more recent and proper way.

This will enable anybody to actually provide gpio configuration for a
winc1500 out of any board specific location.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>