sched/fair: Skip LLC nohz logic for asymmetric systems

The LLC nohz condition will become true as soon as >=2 CPUs in a
single LLC domain are busy. On big.LITTLE systems, this translates to
two or more CPUs of a "cluster" (big or LITTLE) being busy.

Issuing a nohz kick in these conditions isn't desired for asymmetric
systems, as if the busy CPUs can provide enough compute capacity to
the running tasks, then we can leave the nohz CPUs in peace.

Skip the LLC nohz condition for asymmetric systems, and rely on
nr_running & capacity checks to trigger nohz kicks when the system
actually needs them.

Suggested-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>