PM / devfreq: devm_kzalloc to have dev pointer more precisely

devm_kzalloc of devfreq's statistics data structure has been
using its parent device as the dev allocated for.
If a device's devfreq is disabled in run-time,
such allocated memory won't be freed.

Desginating more precisely with the devfreq device
pointer fixes the issue.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>