There is no such thing as "Earth Citizenship" so scratch that one. Nation of origin, hmmm, well okay for first generation, I would assume that would be the legal precedent, lacking any local governmental body to which you could apply for a change in citizen ship, you should retain your original one. Second generation? Now we have a problem.
Hopefully, by the time there IS a second generation on Mars, there WILL be local government. If the local government is a recognized State or Territory of an Earthbound government, again, I see no problem (other than the usual political squabbles that is). The one problem I foresee, is that "Corporate Governmental Entities" (you heard it here first), should and *must* be banned absolutely by International Law PRIOR to this happening.