parse-tpr3-results.rb: Use the text labels when combining results

Group results together only if they have identical labels.
Also the sorting order is now determined primarily by the text
labels. These labels are taken from '_description.txt' files in
the directories with the tpr3 results data.
scan-for-best-tpr3.rb: Use the SoC type as part of the directory name

This is needed, for example, to ensure that the results from
the 'sun4i' Cubieboard1 do not mix with the results from the
'sun7i' Cubieboard2 even if they use identical dram settings.
Retire a10-watchdog