I don't know actually, this isn't really described anywhere. But I think the main conditions are:
- the platfrom isn't super outdated and commonly available and expected to remain so for like a decade (just a wild guess, but I assume we don't want a port for a platform which the manufacturer will stop producing next year or so)
- multiple people can work on it / fix it when needed i.e. have the toolchain and whatever required
- there is enough interest from others i.e. people actually using it
- bonus if it it can be built by the Continuous Integration so it can be verified it at least compiles
Again, this is just what I assume. If you want to get input from the maintainer(s), just create a PR I'd say (after having read CONTRIBUTING.md etc so it's in the correct shape)