i2c: mux: create symlink to actual mux device

The current implementation creates muxed i2c-<n> busses as immediate
children of their i2c-<n> parent bus. In case of multiple muxes on one
bus, it is impossible to determine which muxed bus comes from which mux.

It could be argued that the parent device should be changed from the
parent adapter to the mux device. This has pros and cons. To improve the
topology, simply add a "mux_device" symlink pointing to the actual
muxing device, so we can distinguish muxed busses. Doing it this way, we
don't break the ABI.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>