PM / OPP: add missing of_node_put() for of_get_cpu_node()

Commit 76279291 (PM / OPP: Fix get sharing CPUs when hotplug
is used) moved away from using cpu_dev->of_node because of some
limitations.

However, commit 7467c9d9 (of: return of_get_cpu_node from
of_cpu_device_node_get if CPUs are not registered) added support to
fall back to of_get_cpu_node() if called if CPUs are not registered
yet.

Add the missing of_node_put() for the CPU device nodes. Also go back
to using of_cpu_device_node_get() in dev_pm_opp_of_get_sharing_cpus()
to avoid scanning the device tree again.

Acked-by: Viresh Kumar <vireshk@kernel.org>
Fixes: 76279291 (PM / OPP: Fix get sharing CPUs when hotplug is used)
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>