stand/ofw: Subclass devnet to cope with ofw's unique needs

We need to match devices in a slightly special way: We have to look up
the path and see if the device is a 'network' device in order to use it.

Sponsored by: Netflix
Tested by: grehan@ (with tweaks to my original patch)
Differential Revision: https://reviews.freebsd.org/D37557

(cherry picked from commit b8ff248f6595067ef9a31d5d4cec5fb9b9052fc3)