powerpc/kexec_file: add missing of_node_put

for_each_node_with_property performs an of_node_get on each
iteration, so a break out of the loop requires an
of_node_put.

This was done using the Coccinelle semantic patch
iterators/for_each_child.cocci

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230907095521.14053-7-Julia.Lawall@inria.fr