Add OF_xref_from_device() so that there's no need to have an intermediate
call to ofw_bus_get_node() to lookup info that's already in the xreflist.
Use OF_xref_from_device(), not OF_xref_from_node().  Also, use bzero()
rather than memset().
Fix typo.

Pointy hat:	ian
Update a comment to reflect the changes in r213408.

MFC after:	5 days