cpufreq: maple: fix possible object reference leak

The call to of_cpu_device_node_get 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/maple-cpufreq.c:213:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 177, 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: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>