gpio: use class_groups instead of class_attrs

The class_attrs pointer is long depreciated, and is about to be finally
removed, so move to use the class_groups pointer instead.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: <linux-gpio@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>