arm: Set SD_SHARE_CAP_STATES sched_domain flag when applicable

Energy-aware scheduling relies on the SD_SHARE_CAP_STATES to identify
sharing the same clock source/frequency domain. The assumption is that a
sched_domain exists that match the clock/frequency domain, i.e.
policy->related_cpus in cpufreq terms. The flag is not set for systems
without frequency scaling or systems with per-cpu frequency scaling.

cc: Russell King <linux@armlinux.org.uk>

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