media: v4l: i2c: Regroup lens drivers under their own section

The lens drivers had ended up under the video decoder section; add a new
one just for them, between the camera sensors and flash drivers.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>