After cherry picking a few dozen commits of the brilliant mythbk and following his suggestions I was able to get it up and running.
https://github.com/archfan/coreboot/commit/bd3e5cf5c8277dbb286ec4234e0355afe...
1600MT/s is what the controller officially supports. Anything faster is
overclocking and the mrc.bin likely does not support that.
Correct. There is a hard coded limit of 1600 MT/s in the mrc.bin.