arm64: dts: renesas: r8a7796: salvator-xs: Convert to new LVDS DT bindings

As of commit 6d2ca852 ("dt-bindings: display: renesas: Deprecate
LVDS support in the DU bindings"), the internal LVDS encoder has DT
bindings separate from the DU.  The device trees for all R-Car H3 and
M3-W development boards were ported over to the new model, but
Salvator-XS boards equipped with an R-Car M3-W SoC were forgotten.

Fixes: 58e8ed2e ("arm64: dts: renesas: Convert to new LVDS DT bindings")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>