Reuse the string machine from the device tree data structure instead
of duplicating the data again.  This also prevents a potential memory
allocation failure that was not handled previously.  Also fixes an early
put of root device node.
Reported-by: NNicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: NLi Yang <leoyang.li@nxp.com>