Input: add binding documentation for imx25 touchscreen controller

This is the touchscreen conversion queue binding documentation. It uses
the shared imx25 ADC.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>