cpuquiet: prevent runnable_threads from using many cores at low freqencies

The CPU now has to run at at least 900MHz before runnable_threads can bring a new core online.
This does not reduce the performance, but improves the batterylife during light tasks (such as reading a book or browsing the web) significantly.

Change-Id: I633d322d7738bbe4dc62f79f88d70069fb721e39
fix freq for non-werewolf kernels

Change-Id: I2ba6d65cd089721176500294c68372028b986755