Input: ad7879 - allow exporting AUX/VBAT/GPIO pin via device property

Up until now only platforms using legacy platform data were able to switch
AUX/VBAT/GPIO pin in GPIO mode and use it as regular GPIO line. Let's
allow platforms using generic device properties to do the same.

Reviewed-by: Michael Hennerich <michael.hennerich@analog.com>
Tested-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>