[media] v4l2-subdev.h: add g_tvnorms video op

While there was already a g_tvnorms_output video op, it's counterpart for
video capture was missing. Add it.

This is necessary for generic bridge drivers like soc-camera to set the
video_device tvnorms field correctly. Otherwise ENUMSTD cannot work.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>