sched/debug: Add capacity procfs interface

This patch makes the capacity data available via procfs. The related files
are placed as sub-directory named 'capacity' inside the
/proc/sys/kernel/sched_domain/cpuX/domainY/groupZ directory for those
cpu/domain/group tuples which have energy information.

This is same implementation as Dietmar's "sched/debug: Add energy procfs
interface" (i.e., I stole his patch).

Stolen-from: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>