Use upstream prjxray as submodule

[unbtorsten]

Can we use upstream prjxray in .gitmodules, i.e. url = git@github.com:f4pga/prjxray.git? Note, kintext-chatter:nextpnr-xilinx has its own submodule of a customized repo::branch prjxray-db::k325. This is just about the toolchain.

prjxray in kintex-chatter is ahead of jrrk2:prjxray, which is not aligned with f4pga:prjxray, so I'm hesitant to make this change without comment from the maintainers of the kintex-chatter:prjxray repo.

Originally posted by @rwhitby in #25 (comment)

[hansfbaier]

Not for the high speed IO banks, which needs the iob18-trial branch and it still will take a while until those are fully working.