ARM: dts: imx7-colibri: specify backlight GPIO

The Colibri standard defines SODIMM 71 as backlight enable
GPIO. Assign the GPIO to the backlight node in the module
level device tree.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>