nitrogen6x: add support for Nitrogen6_Max and Nitrogen6_Lite Device Trees

Include the Device Tree binaries for Nitrogen6_Lite and Nitrogen6_Max
to avoid building a completely separate image for each.

Since the Nitrogen6_Lite device tree isn't built by the stock image
in meta-fsl-arm-extra, append it to the KERNEL_DEVICETREE variable.

Change-Id: I085e0473773c84308cb0d826eb8d2da444036ea5
Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@digia.com>