staging: media: imx: Make imx_media_of_add_csi() static

The imx_media_of_add_csi() function is only called in its compilation
unit. Make it static and don't export it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>