When registering an association between a device and an xref phandle, create
an entry in the xref list if one doesn't already exist for the given handle.

On a system that uses phandle properties, the init-time scan of the tree
which builds the xref list will pre-create entries for every xref handle
that exists in the data.  On systems where the xref and node handles are
synonymous there is no phandle property in referenced nodes, and the xref
list will initialize to an empty state.  In the latter case, we still need
to be able to associate a device_t with an xref handle, so we create list
entries on the fly as needed.  Since the node and xref handles are
synonymous, we have all the info needed to create a list entry at device
registration time.

The downside to this change is that it basically allows on the fly creation
of xref handles as synonyms of node handles, and the association of a
device_t with them.  Whether this is a bug or a feature is in the eye of
the beholder, I guess.