ARM: dts: stout: 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 Lager device tree was ported over to
the new model, but the Stout device tree was forgotten.

Fixes: 15a1ff30 ("ARM: dts: r8a7790: Convert to new LVDS DT bindings")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>