PowerCap: Convert class code to use dev_groups

The newly added power capping framework uses the obsolete .dev_attrs
field of struct class. However this field will be removed in 3.13, so
convert the code to use the .dev_groups field instead.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>