Add a kernel interface (OF_xref_phandle()) for systems where phandles

Add a kernel interface (OF_xref_phandle()) for systems where phandles
used as cross-references in the device tree and phandles as used by the
Open Firmware client interface are in different namespaces. This include
IBM pSeries hardware as well as FDT systems. FDT certainly abuses
ihandles for this purpose and should be modified to use this API
eventually. This changes no behavior on systems where FreeBSD already
worked.

Reviewed by:	marius
Approved by:	re (kib)
MFC after:	2 weeks