sched/{fair,cpufreq_sched}: add reset_capacity interface

When a CPU is going idle it is pointless to ask for an OPP update as we
would wake up another task only to ask for the same capacity we are already
running at (utilization gets moved to blocked_utilization).  We thus add
cpufreq_sched_reset_capacity() interface to just reset our current capacity
request without triggering any real update.  At wakeup we will use the
decayed utilization to select an appropriate OPP.

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

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