media: Documentation: ipu6: remove the dma_ops part from the doc

dma_ops override is not in code anymore, so remove it.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>