cpufreq: intel_pstate: Populate the cpu_capacity sysfs entries

Intel hybrid processors have CPUs of different capacity. Populate the
interface /sys/devices/system/cpu/cpuN/cpu_capacity.

This interface uses the per-CPU variable `cpu_scale`. On x86 this
variable has no other use besides feeding the sysfs entries. Initialize
it when setting CPU capacity for the scheduler and scale-invariant code.
Feed it with arch_scale_cpu_capacity() as it gives capacity normalized
to the interval [0, SCHED_CAPACITY_SCALE].

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>