Import OpenBSD's iwm WiFi driver for Intel 3160/7260/7265.

There are still several bugs, but I've been using it for a while now.
Thanks to all the testers and to Adrian for his help with this
driver.

This driver isn't connected to the build yet, but it will be soon.

There's no MFC planned because the driver isn't very stable yet.

Reviewed by: adrian
Obtained from: https://github.com/rpaulo/iwm
Tested by: adrian, gjb, dumbbell (others that I forgot).
Relnotes: yes