Wild birds eat the holly berries, but they usually wait until there's nothing else handy. The reason is they taste terrible. So, while the berries may be only mildly toxic, your chickens may not consider them worth the effort since there are likely many other things in their daily range that taste far better.
If I had a holly tree, I probably would clean up any fallen berries just so as not to take the chance on any chicken deciding it might be a splendid idea to gorge on them. That might cause problems. I certainly wouldn't take the chance.