Cite submitted paper in my master's thesis
Perhaps you can use time to your advantage here and update a citation as you learn more. But submitted, not yet published is a better description than simply unpublished which can give the impression that it won't be published. 

But if the thesis itself will only be seen internally at the university until the decision is made by the conference, then it might be safe enough to just give the full citation. And, if it is accepted you would probably want to update it in any case to indicate that fact. 

I think you understand already that citation isn't optional and must be done, but only the form of citation is being addressed here. Above all else, avoid self plagiarism.