dt-bindings: input: touchscreen: add bindings for eeti touchscreen controller

Describe the bindings for EETI touchscreen controllers.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>