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>