[POWERPC] Fix OF node refcnt underflow in 836x and 832x platform code

Incorrect use of of_find_node_by_name() causes of_node_put()
on a node which has already been put.  It causes the refcount of
the node to underflow, which triggers the WARN_ON in kref_get
for 836x and 832x.  This fixes it.

Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>