PowerBI reports from SharePoint online / Office 365

I don't think there is anything readymade for this, though someone will hopefully correct me if there is. Best I can tell you can only get data in PowerBI from SharePoint lists & folders to report on out of the box.

What you can look out for though is the https://blogs.office.com/2016/10/10/announcing-the-preview-of-the-office-365-adoption-content-pack-in-power-bi/.

This includes numerous reports including SharePoint Usage and Top Sites/Users

SharePoint Usage report —Shows how SharePoint team sites and groups sites are being used to store files and for collaboration. The report also includes information about how many SharePoint sites are actively being used, and how many files are stored on average.

SharePoint - Top Sites /Users - Shows details on sites including document count, disk space used, active users, stats on sharing and syncing etc.  Also shows file activity on SharePoint sites by user.

This will be available around March, it's in closed preview currently.  You can get a good overview of Office 365 reporting from this Ignite session -

Understand your users: what’s new in Office 365 Usage Reporting