You want to "Blanch" rather than "Cook" turnip (greens or roots) before freezing. Cooking time for turnips does not take as long as it does say for Collard. When blanching, be sure to drain off as much of the water as possible before bagging the turnip greens.
I cook and freeze turnips regularly. I think turnips taste better if cooked in a larger volume and since I cook for only 2 I go ahead and cook my large batch, seasoning as I like and then what is left over I freeze in meal size portions. I do like to let them thaw in the refrigerator when I get ready to use them. I also do turnip greens this way (or any greens that I cook).