mfd: Add DT binding documentation for imx25 ADC/TSC

This documentation describes the devicetree bindings for the
ADC/Touchscreen unit of the i.MX25 SoC.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>