Input: axp20x-pek - switch to using devm_device_add_group()

Now that we have proper managed API to create device attributes, let's
use it instead of installing a custom devm action.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>