dt-bindings: display: renesas: lvds: Document r8a77995 bindings

The D3 (r8a77995) supports two LVDS channels. Extend the binding to
support them.

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