dt-bindings: input: touchscreen: ad7879: generic node names in example

Update example in ad7879 device tree documentation to use generic touch
controller node names.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191026090403.3057-3-marcel@ziswiler.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>