I think the explanation "spiritual" could be correct. Indonesia being an Islamic country the word is probably a loan from the Arabic ruhani, same meaning. Internet references to Rohani give the impression that the surname is Iranian. For some reason Muslim names which would be spelt with a U in Roman script when referring to Arabs are usually spelt with an O when referring to Iranians; e.g., Abdullah becomes Abdollah, Hussein becomes Hossein. So Arabic Ruhani would become Rohani. I suppose it could have been used as a forename and so become a patronymic. Iranians only adopted surnames in 1929 by government decree, and I think some of the choices of name would be difficult to explain.