hwmon: (mcp3021) add devicetree bindings documentation

Document the devicetree bindings for the Microchip MCP3021/3221.

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>