powerpc/cell/iommu: Add missing of_node_put

powerpc/cell/iommu: Add missing of_node_put

np is initialized to the result of calling a function that calls
of_node_get, so of_node_put should be called before the pointer is dropped.

The semantic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@
expression e,e1,e2;
@@

* e = \(of_find_node_by_type\|of_find_node_by_name\)(...)
  ... when != of_node_put(e)
      when != true e == NULL
      when != e2 = e
  e = e1
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>