drm/bridge: cdns-dsi: Create a header file

Create a header file for cdns dsi and move structure definations to
prepare for adding j721e wrapper support

Signed-off-by: Rahul T R <r-ravikumar@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230103101951.10963-5-r-ravikumar@ti.com