iio: Add binding documentation for imx25 GCQ

The documentation describes the bindings for the imx25 GCQ unit which is
essentially a generic conversion queue using the imx25 ADC.

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