I know there are a few States that allows you to get a license as long as you go through a year specialty program. Most times it has to be a 2 year ADA program and you are allowed to work only in that one specialty
I am not 100% sure but Texas might be an exception. I believe it recognizes DPH as an specialty and might even allow you to work as a GP and not only in PH.
Double check with their board....