dt-bindings: display: renesas: lvds: Document r8a77965 bindings

The M3-N (r8a77965) includes one LVDS encoder. Extend the binding to
support it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>