ide-generic: add ide_generic class and attribute for adding new interfaces

* Add ide_generic_sysfs_init() helper registering 'ide_generic' class
  (together with ide_generic_class_release() ->class_release method)
  and use it in ide_generic_init().

* Add "add" class attribute to 'ide_generic' class for adding new interfaces
  (it is intended to be a replacement for obsoleted "idex=base[,ctl[,irq]]"
  kernel parameters).

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>