media: imx: Unstage the imx8mq-mipi-csi2 driver

The imx8mq-mipi-csi2 MIPI CSI-2 receiver driver is used and maintained.
There is no reason to keep it in staging. The accompanying CSI bridge
driver that uses it is in drivers/media/platform/nxp as well.

One TODO is to get rid of csi_state's "state" and "lock" variables.
Especially make sure suspend/resume is working without them. That can
very well be worked on from the new location.

Also add a MAINTAINERS section for the imx8mq-mipi-csi2 mipi receiver
driver.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>