Input: ad7846 - move bindings to touchscreen subdirectory

This moves device tree bindings for ADS7846 together with the other
touchscreen bindings.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>