ofwbus: only allow unit number zero

ofwbus has always been the root of attachment for OFW/FDT platforms. It
may have simplebus children, but we expect only one instance of the
ofwbus driver, added directly by nexus. We may as well ensure this
remains the case.

Reviewed by: jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D38493