sched: Task wakeup tracking

Track task wakeup rate in wakeup_avg_sum by counting wakeups. Note that
this is _not_ cpu wakeups (idle exits). Task wakeups only cause cpu
wakeups if the cpu is idle when the task wakeup occurs.

The wakeup rate decays over time at the same rate as used for the
existing entity load tracking. Unlike runnable_avg_sum, wakeup_avg_sum
is counting events, not time, and is therefore theoretically unbounded
and should be used with care.

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>