drivers: cpufreq: Send a uevent when governor changes

* Useful so userspace tools can reconfigure.

Change-Id: Ib423910b8b9ac791ebe81a75bf399f58272f64f2

idea is taken here -
https://github.com/poondog/kangaroo-m7-mkIII/commit/aa9a0d38af620565f287a6bef39527b87d6800be