[media] media: Add userspace header file for i.MX

This adds a header file for use by userspace programs wanting to interact
with the i.MX media driver. It defines custom events and v4l2 controls for
the i.MX v4l2 subdevices.

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>