ARM: dts: imx7-colibri: use NAND_CE1 as GPIO

All Colibri iMX7 SKUs use either single-die NAND or eMMC, hence
NAND_CE1 is not used on the module and can be used as a GPIO by
carrier boards.

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