A list of flowers that are edible for people would probably be safe for chickens too.Also, I would assume that the flowers would have to be non-toxic to the chickens in case they try to eat them.
In addition to the flowers other people mentioned, I know pansies are edible, and they tend to be easy to find anywhere that sells started plants. They tend to be short plants (so they don't fall over, but there's a chance they might get buried by what the chickens scratch out). They typically grow and bloom for a few months and then die. Some varieties will drop seeds that grow the next year, some do not.