driver core: make device_{add|remove}_groups() public

Many drivers create additional driver-specific device attributes when
binding to the device. To avoid them calling SYSFS API directly, let's
export these helpers.

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