sched: Introduce SD_SHARE_CAP_STATES sched_domain flag

cpufreq is currently keeping it a secret which cpus are sharing
clock source. The scheduler needs to know about clock domains as well
to become more energy aware. The SD_SHARE_CAP_STATES domain indicates
whether cpus belonging to the domain share capacity states (P-states).

There is no connection with cpufreq (yet). The flag must be set by
the arch specific topology code.

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