dt-bindings: display: bridge: thc63lvd1024: Document dual-output mode

The DT binding support both dual-input and dual-output mode, but only
dual-input is documented. Document dual-output mode.

Suggested-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>