media: imx: Decouple imx8mq-mipi-csi2 from imx7-media-csi

The imx8mq-mipi-csi2 driver targets SoCs that also run the
imx7-media-csi driver, but they are distinct. Decouple them in Kconfig
to prepare for destaging of the imx7-media-csi driver.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>