Fluch cache on all sites in multisite platform
An easy way is via the views bulk operations.
When you select some or all sites you can expand the operations menu at the top, where you should find 'Site: Flush_cache'. It's part of the hosting_tasks_extra module (which is included by default now)

Note that you currently do have to enable the operation in the sites view. See https://www.drupal.org/node/2715945