[media] v4l2-subdev: add support for the new enum_frame_interval 'which' field

Support the new 'which' field in the enum_frame_interval ops. Most drivers do not
need to be changed since they always returns the same enumeration regardless
of the 'which' field.

Tested for ov7670 and marvell-ccic on a OLPC XO-1 laptop.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>