cpufreq: cpu-boost: Export cpu boost status

* This will help hotplug drivers to know boost status
  of CPU and act accordingly.

Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>
dyn_hotplug: Do not take down boosted CPU

* Check whether CPU is boosted or not before taking it down.
* Wait for next cycle if CPU is currently being boosted.

Adapted from neobuddy89 idea