media: imx7.rst: add documentation for i.MX7 media driver

Add rst document to describe the i.MX7 media driver and also a working
example from the Warp7 board usage with a OV2680 sensor.

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>