How do I create DNS entries for EC2 instances created by Auto Scaling?
This is not something you can do with CloudFormation, as its involvement stops at defining the auto-scaling groups - it doesn't got to see the instances started by the ASG. Auto-scaling groups don't give you any way to do this either.
Instead, you could ensure your instances run something on startup to register themselves in Route 53. This post talks about using Chef to do it, but you could do the same thing in a standalone script.