media: imx: imx-mipi-csis: Implement the .get_frame_desc() operation

The CSIS is connected to its sink through an SoC-specific gasket that
needs to be configured. Depending on the platform, the gasket
configuration requires knowing the CSI-2 DT. To provide the needed
information, implement the .get_frame_desc() operation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>