drivers: cpufreq: Send a uevent when governor changes

 * Useful so userspace tools can reconfigure.

Change-Id: Ib423910b8b9ac791ebe81a75bf399f58272f64f2
cpufreq: Fix broken uevents for cpufreq governor and cpu devices

cyanogens uevent commit was rendered non-working since kitkat (or so)
as the uevent filter function caused events to be dropped. Create an
empty filter to cure this. Then hook into cpufreq_core_init() and
cpufreq_add_dev_interface() to create our ksets for cpufreq and cpu
devices.

This commit requires a change in ueventd.rc that add rules for several
files of interest.

Change-Id: I3aafa0d4e18363e1d68535f513099ecd27024007