iio: adc: set INPUT_PROP_DIRECT

Set INPUT_PROP_DIRECT to indicate that it is a touchscreen on the
device to help userspace classify it.

Signed-off-by: Ksenija Stanojevic <ksenija.stanojevic@gmail.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>