ARM: dts: imx6dl-yapp4: Bind the backlight controller to the LCD panel

Add connection between the backlight controller and LCD panel.
With that the backlight is automatically switched on when the panel
is on or switched off when the panel is blanked.

Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>