ARM: dts: imx: add Boundary Devices Nitrogen6_SOM2 support

SoM based on i.MX6 Quad with 1GB of DDR3.

https://boundarydevices.com/product/nit6x-som-v2/



Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>