[media] MAINTAINERS: add entry for Freescale i.MX media driver

Add maintainer entry for the imx-media driver.

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>