driver core: add device_{add|remove}_group() helpers

We have helpers that work with NULL terminated array of groups, but many
drivers only create a single supplemental group, and do not want to declare
a group array. Let's provide them with helpers working with a single group.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>