cpupower: Add support for setting EPP via systemd service

cpupower: Add support for setting EPP via systemd service

Extend the systemd service so that it can be used for tuning the Energy
Performance Preference (EPP) as well. Available options can be read from
/sys/devices/system/cpu/cpufreq/policy0/energy_performance_available_preferences.
The desired one can then be set in cpupower-service.conf.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>