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.