tools/power turbostat: add format "average" for external attributes

External atributes with format "raw" are not printed in summary lines
for nodes/packages (or with option -S). The new format "average"
behaves like "raw" but also adds the summary data

Signed-off-by: Michael Hebenstreit <michael.hebenstreit@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>