drivers/wifi: Add winc1500 WiFi driver

Adding support for WINC1500 WiFi chip.

It introduces the wifi drivers sub-directory.

It provides a Full-MAC for 802.11 and an offloaded network stack as
well. The driver uses Atmel's winc1500 HAL.

Signed-off-by: Dario Pennisi <dario@iptronix.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>
Signed-off-by: Massimiliano Agneni <massimiliano.agneni@iptronix.com>
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>