sched/cpufreq_sched: use static key for cpu frequency selection

Introduce a static key to only affect scheduler hot paths when sched
governor is enabled.

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>

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