phodevi: Add Radeontop support for getting proper GPU usage on FOSS radeon

Sample results posted at http://openbenchmarking.org/result/1404174-KH-RADEONTOP86

Radeontop is packaged in Debian, Ubuntu, Arch, and possibly others. It does require root rights, so if you're running PTS as an user, I recommend to set radeontop as suid root beforehand.

I'm not sure if it's supported in PTS to install packages automatically for sensors.

Looking forward to having GPU usage in the future Catalyst vs FOSS articles.