power_key.py: make compatible with 5.6 and later and the devkit
this is when we use linux-next!105 (closed) for 5.6.
devfreq is being handled differently on our kernels. With 5.6+ not needing the script anymore, for devfreq, when the powersave governor is set by udev. Since we don't want to assume this yet, we set it.
This also makes devfreq settings compatible with the devkit again.