power_key.py: make compatible with 5.6 and later and the devkit
devfreq is being handled differently on our kernels. With 5.6+ frequency is switched automatically by the kernel for devfreq, when the powersave governor is set. So in that case we only to that (done by udev in the future). This also makes devfreq settings compatible with the devkit again because it reads the minimum frequency which is different than on the phone, from sysfs. Also, we particularly handle 5.6-rc based kernel versions that include a broken devfreq implementation / setup. starting with version 5.6.0 we need to increment the ABI version to 2.