[media] davinci: vpfe: Add documentation and TODO

Add documentation on the Davinci VPFE driver. Document the subdevs,
and private IOTCLs the driver implements. This patch also includes
the TODO's to fit into drivers/media/ folder.

Signed-off-by: Manjunath Hadli <manjunath.hadli@ti.com>
Signed-off-by: Lad, Prabhakar <prabhakar.lad@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>