net: l2: Add support to identify Wi-Fi interfaces

Wi-Fi is based on L2 Ethernet, so, all drivers are registered as
Ethernet L2, but in order to distinguish true Ethernet and Wi-Fi
devices, add a L2 type within Ethernet.

Also, handle offloaded net devices that also offload Wi-Fi.

This approach is better than adding a new Wi-Fi L2 as that would mean
invasive changes which are unnecessary.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>