vTPM: fix searching for the right vTPM node in device tree

Replace all occurrences of '/ibm,vtpm' with '/vdevice/vtpm',
as only the latter is guanranteed to be available for the client OS.
The '/ibm,vtpm' node should only be used by Open Firmware, which
is susceptible to changes.

Signed-off-by: Hon Ching(Vicky) Lo <honclo@linux.vnet.ibm.com>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>