ARM: dts: imx6qdl-nitrogen6x: Add SPI NOR partitions

This adds the partitions definition for the SPI NOR to provide
backward compatibility with the documented[1] layout used with
Boundary Devices BSP.

1. https://boundarydevices.com/boot-flash-access-linux/



It exports to Linux:

 mtd0: bootloader
 mtd1: env
 mtd2: splash

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>