Well you need to use a func_door_rotating.
Create the door that will rotate. Then create a small brush where your hinges will be. texture the brush with "origin". And keep it a simple square. Select the main door part and the origin and group it together, then assign the "func_door_rotating" entity to it. You should end up with a rotating door that rotates wherever you put the origin brush. You can change the distance it moves, etc... You might need to change the axis it rotates around. Just mess around with the properties and look them up online.