cpufreq: pmac32-cpufreq: Fix refcount leak bug

In pmac_cpufreq_init_MacRISC3(), we need to add corresponding
of_node_put() for the three node pointers whose refcount have
been incremented by of_find_node_by_name().

Signed-off-by: Liang He <windhl@126.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>