of/promtree: add package-to-path support to pdt

package-to-path is a PROM function which tells us the real (full) name of the
node.  This provides a hook for that in the prom ops struct, and makes use
of it in the pdt code when attempting to determine a node's name.  If the
hook is available, try using it (falling back to looking at the "name"
property if it fails).

Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>