RFC: Provide mainline Linux support

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The current device tree magic can only generate device trees for Xilinx downstream kernels. If you want to create a device tree for a custom device with upstream compatibility, you're in a lost course right now.

I've hacked up the current code base to emit an upstream compatible device tree instead, but this obviously breaks downstream support.

I'm more than happy to receive feedback on how to properly integrate mainline support into the Xilinx tree, so that you could also generate an upstream compatible device tree using Xilinx tools.