PowerCap: Documentation

Added power cap framework documentation. This explains the use of power
capping framework, sysfs and programming interface.

There are two documents:
 - Documentation/power/powercap/powercap.txt : Explains use case and APIs.
 - Documentation/ABI/testing/sysfs-class-powercap: Explains ABIs.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>