linux-tools: add cpupower

This patch is based on the patch send by James Knight:
http://lists.busybox.net/pipermail/buildroot/2015-May/128754.html



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Cc: James Knight <james.knight@rockwellcollins.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>