tools: cpupower: Add cpupower-idle-set(1) manpage

The cpupower idle-set subcommand was introduce recently.
This patch provides the missing manpage.

If cpupower is properly installed it will show up automatically
(similar to git), when invoking:
cpupower help idle-set
or
cpupower idle-set --help

Some parts have been taken over and adjusted from
git commit 62d6ae88
documentation submitted by Carsten Emde.

Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>