FROMLIST: sched/core: Disable SD_ASYM_CPUCAPACITY for root_domains without asymmetry

When hotplugging cpus out or creating exclusive cpusets systems which
were asymmetric at boot might become symmetric. In this case leaving the
flag set might lead to suboptimal scheduling decisions.

The arch-code proving the flag doesn't have visibility of the cpuset
configuration so it must either be told by passing a cpumask or by
letting the generic topology code verify if the flag should still be set
when taking the actual sched_domain_span() into account. This patch
implements the latter approach.

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
[from https://lore.kernel.org/lkml/1530699470-29808-12-git-send-email-morten.rasmussen@arm.com/

]
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Change-Id: I6a369e474743c96fe0b866ebb084e0a250e8c5d7