Input: gpio_keys - convert driver to use dev_groups

Drivers now have the option to have the driver core create and remove any
needed sysfs attribute files.  So take advantage of that and do not
register "by hand" a bunch of sysfs files.

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