Input: ad7877 - use attribute group to control visibility of attributes

Instead of manually creating one set of attributes or another set up
is_visible method in attribute group structure to control whether
aux3 or gpio3 attribute is presented to userspace.

Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>