ARM: dts: imx7-colibri: add NAND support

The Colibri iMX7 modules come with 512MB on-module SLC NAND flash
populated. Make use of it by enabling the GPMI controller.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Han Xu <han.xu@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>