tools/power/turbostat: Abstrct function for parsing cpu string

Abstract parse_cpu_str() which can update any specified cpu_set by a
given cpu string. This can be used to handle further CPU limitations
from other sources like cgroup.

The cpu string parsing code is also enhanced to handle the strings that
have an extra '\n' before string terminator.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>