Go to Eclipse → Preferences → Maven → Discovery and click “Open catalog”. You should find a list of extra m2e connectors, including a connector for Tycho.
This should automatically add an update site. In my case it is:
https://repo1.maven.org/maven2/.m2e/connectors/m2eclipse-tycho/0.8.1/N/0.8.1.201704211436/
but I don’t think you should try to add it manually.