media: staging/imx7: add imx7 CSI subdev driver

This add the media entity subdevice and control driver for the i.MX7
CMOS Sensor Interface.

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>
[hverkuil-cisco@xs4all.nl: clean up some alignment warnings]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>