luci-app-statistics: cpufreq: tweak graphs

* Frequency usage percentage stats are by kHz instead of Hz.
  Correct the labels. (Linux natively uses kHz for CPU frequency stats,
  but collectd scales the current frequency stats item to Hz.)

* Show frequency usage percentage graph before the transition counts,
  as it is more descriptive.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>