i2c: muxes are not EXPERIMENTAL anymore

We got multiple patches to add mux support to device tree, so people are
using it happily already and build up on it. I also used it in a project
without encountering problems. 20 months of EXPERIMENTAL should do for
this.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Michael Lawnick <ml.lawnick@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: David Daney <ddaney.cavm@gmail.com>