media: MAINTAINERS: add entry for Freescale i.MX7 media driver

Add maintainer entry for the imx7 media csi, mipi csis driver,
dt-bindings and documentation.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>