i2c/busses: Add support for Diolan U2C-12 USB-I2C adapter

This patch adds support for the Diolan U2C-12 USB-I2C adapter.
It also updates MAINTAINERS to list the author as maintainer.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>