sched: Consider a not over-utilized energy-aware system as balanced

In case the system operates below the tipping point indicator,
introduced in ("sched: Add over-utilization/tipping point
indicator"), bail out in find_busiest_group after the dst and src
group statistics have been checked.

There is simply no need to move usage around because all involved
cpus still have spare cycles available.

For an energy-aware system below its tipping point,  we rely on the
task placement of the wakeup path. This works well for short running
tasks.

The existence of long running tasks on one of the involved cpus lets
the system operate over its tipping point. To be able to move such
a task (whose load can't be used to average the load among the cpus)
from a src cpu with lower capacity than the dst_cpu, an additional
rule has to be implemented in need_active_balance.

Change-Id: I02aa66bcdd10659ff2c748239fd8d22c218368e6
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>