Add necessary bits to get FreeBSD booting on the Unifi Security Gateway

Summary:
The Ubiquiti Unifi Security Gateway is virtually identical to the
EdgeRouter Lite, with a smaller PCB and apparently a different board identifier.
Simply adding the new board identifier alongside the ERL identifier, FreeBSD
boots successfully, and can access the needed peripherals (tested with USB
booting, and basic pings on one ethernet interface)

Reviewed By: adrian
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D10639