Input: ad714x - fix up input configuration

Add missing input name and set up parent devices so input devices will
show in proper places in sysfs tree.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Tested-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>