cpufreq: pmac32: fix possible object reference leak

cpufreq: pmac32: fix possible object reference leak

The call to of_find_node_by_name returns a node pointer with refcount
incremented thus it must be explicitly decremented after the last
usage.

Detected by coccinelle with the following warnings:
./drivers/cpufreq/pmac32-cpufreq.c:557:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 552, but without a corresponding object release within this function.
./drivers/cpufreq/pmac32-cpufreq.c:569:1-7: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 552, but without a corresponding object release within this function.
./drivers/cpufreq/pmac32-cpufreq.c:598:1-7: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 587, but without a corresponding object release within this function.

Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linux-pm@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>