Added symbols for MCP41xxx digital potentiometers

I've added symbols for MCP41xxx digital potentiometers to interface.lib

The datasheet is at
http://ww1.microchip.com/downloads/en/DeviceDoc/11195c.pdf

Please verify and suggest any corrections/improvements.

What is the convention for background color? Should it be filled or not?

What is the convention for reference designator? Should it be 'U' or 'IC'?