arm64: dts: imx8mm-kontron: Add SPI NOR partition layout

This is the layout used by the bootloader. Add it to the kernel
devicetree to make the same layout available in Linux and have
the devicetrees synced.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>