iio: potentiometer: mcp4531: Add device tree binding documentation

Add the device tree documentation for all the supported parts. Apart the
compatible string and standard I2C binding, no other binding is currently
needed.

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