Introduce qwx(4), a work-in-progress port of the Linux ath11k driver.

Introduce qwx(4), a work-in-progress port of the Linux ath11k driver.

This driver is not working yet. Scanning almost works but a lot more
work remains to be done. So far most of the porting work was done by
myself, with some help from mpi, patrick, and kettenis.

Obviously this driver remains disabled for now. Enable relevant lines
in the kernel config if you want to help out with development.

At present firmware files must be obtained manually and placed
in the directory /etc/firmware/qwx/WCN6855/hw2.1/
This will be improved later.

Thanks to the OpenBSD Foundation for supporting this effort.