tools/power turbostat: Harden probe_intel_uncore_frequency()

If sysfs directory "intel_uncore_frequency/cluster00/" exists,
then use uncore cluster code (now its own routine).

The previous check for
"intel_uncore_frequency/package_00_die_00/current_freq_khz",
could be unreliable in the face of sparse die id's.

Signed-off-by: Len Brown <len.brown@intel.com>