dt-bindings: drm/bridge: Document Cadence MHDP8546 bridge bindings

Document the bindings used for the Cadence MHDP8546 DPI/DP bridge in
yaml format.

Signed-off-by: Yuti Amonkar <yamonkar@cadence.com>
Signed-off-by: Swapnil Jakhade <sjakhade@cadence.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>