cpufreq: powernv: add of_node_put()

The of_find_node_by_path() returns a node pointer with refcount
incremented,but there is the lack of use of the of_node_put() when
done.Add the missing of_node_put() to release the refcount.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>