dt-bindings: display: renesas: lvds: Document r8a774c0 bindings

The RZ/G2E (r8a774c0) supports two LVDS channels. Extend the binding to
support them.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>