ARM: dts: imx6ul-evk: Pass the 'backlight' property

Pass the 'backlight' property as described in
Documentation/devicetree/bindings/display/panel/simple-panel.txt

Signed-off-by: Marco Franchi <marco.franchi@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>