tools/power turbostat: Read Core-cstates via perf

Reading the counters via perf can be done in bulk with a single syscall,
making the counter values more accurate with respect to one another by
minimizing the time gap between individual counter reads.
Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>