Hi Stuart.
Attached you could find the symbols and footprints for the i.MX6SX in MAPBGA400 package (14x14).
Regarding the DTB, you could use as reference the DTS from the official BSPs, and removing the modules that are not included on the target package, for compiling and generating a proper DTB file.
Hope this will be useful for you.
Best regards!
/Carlos
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