driver core: class: add class_groups support

struct class needs to have a set of default groups that are added, as
adding individual attributes does not work well in the long run.  So add
support for that.

Future patches will convert the existing usages of class_attrs to use
class_groups and then class_attrs will go away.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>