i2c: mv64xxxx: remove EXPERIMENTAL tag

As git history indicates, the driver predates the git era and is heavily
used and worked on since. Not EXPERIMENTAL anymore.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Rodolfo Giometti <giometti@linux.it>
Cc: Florian Fainelli <ffainelli@freebox.fr>