MFC r270953, r270958, r270960, r271190, r271199, r271202:

  Create a mechanism for looking up a device_t associated with an ofw/fdt
  xref handle, and for registering that association.  Also use the same data
  for faster translations between node and xref handles.

  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.

  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.