cpuboost: Add hotplug boosting

If we decide to hotplug a core then we are making a strong
statement that we think that the system load is high.  Let's
have faith in our hotplug driver(s) and boost the cpufreq
when we are hotplugging cores.

[myfluxi: shorter boost, minor style changes]

Change-Id: Id35bae6c27c39097c7aca70eebf1498ed0c3dfdc