The .dts is not board specific, it's design specific. In my opinion,
this is not something that 'a vendor might maintain multiple versions
of': instead it is in most cases simply fundamental to the FPGA design
flow. In fact, in most cases, I'd like to make the .dts file part of
the bitstream and not compiled into the kernel.
Although powerpc has a bit more boot-time complexity than the microblaze
does currently, I think it makes alot of sense to have some consistency
here, and there is already a pattern to follow here which nicely
orthogonalizes multiple .dts files for the same platform code.