iio: adc: max1363: Add device tree binding documentation

Add the device tree documentation for all the supported parts. Mandatory
binding is the compatible string and the slave I2C address.

Optional properties can be used to specify the Vcc / Vref regulators, as
well as the IRQ line if available.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>