cpupower: do not call systemctl at install time

cpupower: do not call systemctl at install time

Fix the installation procedure for the systemd service unit
'cpupower.service'. Do not call "systemctl daemon-reload" in the
Makefile, but explain when this command should be manually issued
in the README file.

Link: https://lore.kernel.org/linux-pm/20250509002206.bd2519ba52035d47c3c32aa6@paranoici.org/T/#mfbb938f9c0d5a21173acb92a061eb9205fd0abfe
Link: https://lore.kernel.org/r/20250513163937.61062-4-invernomuto@paranoici.org
Fixes: 9c70b779ad91 ("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>