stand/kshim: Update for devclass being removed from DRIVER_MODULE

The kshim code abused the devclass argument to DRIVER_MODULE in some
odd ways. Instead, refactor the devclass handling to more closely
mirror what new-bus does in the kernel by having a linked list of
devclasses looked up by name and associate devices with a devclass.

Devices are now only associated with a module while probing and
attaching.

Reviewed by: imp, markj
Differential Revision: https://reviews.freebsd.org/D48409