Turn it into a script_object and add the following key/value to the entity.
key: rotateY
Value: 360
If "Y" does not make it spin the right way, try "X" or "Z". The number is how meny degrees it will rotate per second. A negitive number will make it rotate the other direction. Here is a tutorial map for a ceiling fan.
If you need an object to spin but not in the middle add a brush with the origin texture on it to the script object and it will spin around that . Just add it before you make it a script object or use add to entitiy from the selection menu to do it after .
If you select it and right click on the 2d window and select script ->script_object and add the keys in tltrude's post that is all you will have to do no script needed .
I don't know how you can say it is easier I mean it is func_rotateing door instead of a script_object . For a script_object you don't have to add a origin brush it will spin around it's center . You can add an origin brush if you want to rotate around it instead . Other than that for the rotateing door you have to add a key not player that you don't have to add for a script object . The keys and values are the same for both . So I can't say one is any easier than the other just different . I guess next you will want to make an elevator from a func_door . Key angle value -1 .