cpupower: add a systemd service to run cpupower

One of the most typical use cases of the 'cpupower' utility works as
follows: run 'cpupower' at boot with the desired command-line options
and then forget about it.

Add a systemd service (disabled by default) that automates this use
case (for environments where the initialization system is 'systemd'),
by running 'cpupower' at boot with the settings read from a default
configuration file.

The systemd service, the associated support script and the
corresponding default configuration file are derived from what is
provided by the Arch Linux package (under "GPL-2.0-or-later" terms),
modernized and enhanced in various ways (the script has also been
checked with 'shellcheck').

Link: https://gitlab.archlinux.org/archlinux/packaging/packages/linux-tools/-/tree/dd2e2a311e05413d0d87a0346ffce8c7e98d6d2b



Signed-off-by: Francesco Poli (wintermute) <invernomuto@paranoici.org>
Reviewed-by: John B. Wyatt IV <jwyatt@redhat.com>
Reviewed-by: John B. Wyatt IV <sageofredondo@gmail.com>
Tested-by: John B. Wyatt IV <jwyatt@redhat.com>
Tested-by: John B. Wyatt IV <sageofredondo@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>