iio: potentiometer: mcp4531: Add support for MCP454x, MCP456x, MCP464x and MCP466x

This patch adds support for MCP454x, MCP456x, MCP464x and MCP466x parts.
The main difference with currently supported parts (MCP453x and alike) is
the addition of a non-volatile memory in order to recall the wiper setting
at power-on. This feature is currently not supported and only the
volatile memory is used to set the wiper.

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>