FIXUP: sched/cpufreq_sched: fix hotplug by implementing init/exit

There is a race condition between cpu hotplug and gov_sched stop path
that can result in a OOPS. Also, gov_sched is still lacking proper
implementation for POLICY_{INIT,EXIT} events.

Fix the bug by implementing such methods and by ensuring gov_sched has
always a valid reference to gd->task (via {get,put}_task_struct()).

Signed-off-by: Juri Lelli <juri.lelli@arm.com>