arm64: dts: r8a7795: salvator-x: Add DU LVDS output endpoint

Declaring the endpoint makes LVDS enablement easier by just including
the corresponding panel's dtsi file.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>