Input: ad7879 - support auxiliary GPIOs via gpiolib

Drop the simple fancy sysfs hooks for the aux GPIOs and expose these via
the gpiolib interface so that other drivers can use them.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>