iio: potentiometer: mcp4531: Add device tree binding

This patch adds the necessary device tree binding to allow DT probing of
currently supported parts.

Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>