build(deps): bump litex/deps/migen from 3ffd64c to 35203d6 (#521)

Bumps [litex/deps/migen](https://github.com/m-labs/migen) from `3ffd64c` to `35203d6`.
- [Release notes](https://github.com/m-labs/migen/releases)
- [Commits](m-labs/migen@3ffd64c...35203d6)

---
updated-dependencies:
- dependency-name: litex/deps/migen
  dependency-type: direct:production
...

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