cpupower: do not install files to /etc/default/

Improve the installation procedure for the systemd service unit
'cpupower.service', to be more distro-agnostic. Do not install the
service unit configuration file to /etc/default/ (a directory that
is used by Debian and Debian-derivatives and only rarely by other
distros).

Also, clarify the role of the configuration file in its own comments.

Link: https://lore.kernel.org/linux-pm/20250509002206.bd2519ba52035d47c3c32aa6@paranoici.org/T/#ma8a3fa80acc4036af6c754e8ecabacc55b288ad1

Link: https://lore.kernel.org/r/20250513163937.61062-5-invernomuto@paranoici.org


Fixes: 9c70b779 ("cpupower: add a systemd service to run cpupower")
Signed-off-by: Francesco Poli (wintermute) <invernomuto@paranoici.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>