Add bindings documentation for the i.MX media driver.

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