Extend bcm21553 cpu frequencies & fix cpufreq attribute read/store

* Extend cpufreq driver to allow 156Mhz & 624Mhz frequencies
* Ensure that cpufreq governors can access cpufreq attributes correctly
* Tune cpufreq transition latency to fix ondemand scheduler

Change-Id: I67835fe4043ca709c2c81b97cbb6f245156b687b