cpufreq: ondemand: Use hotplug thread infrastructure

The dbs_sync threads caused considerable load, even when ondemand
wasn't even active. Use the hotplug thread infrastructure to park
the threads.

Change-Id: I8e2c780e3f00b73edc0337e01c226386146e0fd4
cpufreq: ondemand: Remove unused waitqueue

Change-Id: Iee8bb3b319be81fd232e3acf392c9548fec71320