hammerhead: Make SYSTEM owner of cpu online files

This is required for the Settings app to online cpus
before writing to cpufreq files.

Change-Id: I63530bfcf095a0fbc432a908272bc89bb1ad89eb
hh: Better scaling values

 * Default slack is too long and boost is too high, these values
   collide. Fix them.
 * Set a better timer_rate to reduce jitter

Change-Id: Id99c1642ca38d184e75d09bd3afc71951a8801c0