tools/power/turbostat: Introduce cpu_allowed_set

Turbostat supports "-c" parameter which limits output to system summary
plus the specified cpu-set. But some code still uses cpu_present_set to
read and dump the counters.

Introduce cpu_allowed_set for code that should obey the specified cpu-set.

No functional change.

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