media: staging/imx7: add MIPI CSI-2 receiver subdev for i.MX7

Adds MIPI CSI-2 subdev for i.MX7 to connect with sensors with a MIPI
CSI-2 interface.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
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>