ARM: dts: imx6sl-evk: Convert from fbdev to drm bindings

It is preferred to use the panel compatible string rather than passing
the LCD timing in the device tree.

So pass the "sii,43wvf1g" compatible string which describes the parallel
LCD.

Also, 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>