sched: Add sd energy procfs interface

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

The following example depicts the contents of
/proc/sys/kernel/sched_domain/cpu0/domain0/group[01] for a system which
has sd energy information attached to domain level 0.

├── cpu0
│   ├── domain0
│   │   ├── busy_factor
│   │   ├── busy_idx
│   │   ├── cache_nice_tries
│   │   ├── flags
│   │   ├── forkexec_idx
│   │   ├── group0
│   │   │   └── energy
│   │   │       ├── cap_states
│   │   │       ├── idle_power
│   │   │       ├── max_capacity
│   │   │       ├── nr_cap_states
│   │   │       └── wakeup_energy
│   │   ├── group1
│   │   │   └── energy
│   │   │       ├── cap_states
│   │   │       ├── idle_power
│   │   │       ├── max_capacity
│   │   │       ├── nr_cap_states
│   │   │       └── wakeup_energy
│   │   ├── idle_idx
│   │   ├── imbalance_pct
│   │   ├── max_interval
│   │   ├── max_newidle_lb_cost
│   │   ├── min_interval
│   │   ├── name
│   │   ├── newidle_idx
│   │   └── wake_idx
│   └── domain1
│       ├── busy_factor
│       ├── busy_idx
│       ├── cache_nice_tries
│       ├── flags
│       ├── forkexec_idx
│       ├── idle_idx
│       ├── imbalance_pct
│       ├── max_interval
│       ├── max_newidle_lb_cost
│       ├── min_interval
│       ├── name
│       ├── newidle_idx
│       └── wake_idx

The files 'idle_power', 'max_capacity', 'nr_cap_states' and 'wakeup_energy'
contain a scalar value whereas 'cap_states' contains a vector of
(compute capacity, power consumption @ this compute capacity) tuples.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>