tools/power/turbostat: Move process to root cgroup

When available CPUs are reduced via cgroup cpuset controller, turbostat
will exit with errors (For example):
	get_counters: Could not migrate to CPU 0
	turbostat: re-initialized with num_cpus 20
	get_counters: Could not migrate to CPU 0
	turbostat: re-initialized with num_cpus 20

Move the turbostat to root cgroup, which has every CPU.

Writing the value 0 to a cgroup.procs file causes the writing
process to be moved to the corresponding cgroup.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Tested-by: Zhang Rui <rui.zhang@intel.com>