MAINTAINERS: add maintainer for tools/power/cpupower

Based on discussions with Rafael J. Wysocki, cpupower is need of an
active maintainer. I decided to on take the task of maintaining this
tool.

Patches will flow through the pm sub-systems to the mainline.

Suggested-by: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Acked-by: Thomas Renninger <trenn@suse.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>