The gemara learns out many halachos of Aveilus from pesukim in Yechezkel. Yechezkel was told to behave like an avel yet forgo many of the halachos demanded of an avel. We derive from this the types of things that are forbidden for an Avel. For example, Hashem tells Yechezek that he should wear tefillin, wear shoes, and not wrap his head. We learn from here that all these things are forbidden to a regular Avel, just that Yechezkel was supposed to demonstrate that later on the Jews are going to be aveilim yet no one will behave like an avel to mourn for them. The only two things that Yechezkel was told to do in order to behave like an avel was - האנק דום, from which the gemara learns out that he was forbidden in she'eilas shalom and forbidden in talmud torah.
The Rosh (end of Siman 3) writes in the name of Rashi that everything that is learned from Yechezkiel must be d'oraysa because we learn from Yechezkel that we exempt an avel from tefillin (even if it only means one day). How can a pasuk in Yechezkiel override the mitzvah d'oraysa of tefillin, unless we assume that everything learned from Yechezkiel is d'oraysa. The Rosh rejects this and says that perhaps the things that are derive from Yechezkiel are really only d'rabonon, but the Rabbonon have the authority to uproot a d'oraysa so long as it is done בשב ואל תעשה - violated passively.
Tosafos 5a already addresses the issue of whether things learned from Yechzkiel are considered d'oraysa. The gemara learns out tziyun kever, the obligation to mark of graves from pesukim in Yechezkiel, to which the gemara says - מקמי דליתי יחזקאל מאן אמר, אלא גמרא גמירי לה ואתא יחזקאל ואסמכה אקרא. This is also used to explain the source for a mumar being unable to bring a korban - כל ...ערל לב לא יבא אל מקדשי. Tosafos writes that the gemara assumes that anything learned from Yechezkiel has d'oraysa status which compels the question of how it was known prior to Yechezkiel, forcing the gemara to say that it is a halacha l'moshe misinai. On the other hand the gemara in Nida 57a (cited by R. Betzalel Ronsberg) implies that things learned from Yechezkiel are only d'rabonon, and Tosafos in Baba Basra 147a explains that the pesukim in Yechezkiel are only used as an Esmachta. There seems to be a contradiction between Tosafos in MK and Tosafos in BB whether halachos learned from Yechezkiel are assumed to be d'oraysa.
This issue is really a point of debate between the Rambam and Ramban (shoresh sheini of sefer hamitzvos). The Rambam holds that anything introduced by nevi'im after Moshe Rabbeinu is by definition only d'rabonon. The Ramban disagrees and says that there are takanos of Moshe that are only d'rabonon, and there are halachos derived from the nevi'im that are d'oraysa. The Ramban explains that anything written in the nevi'im in the form of a command, to either do or not do, is considered d'oraysa. The approach of the Ramban is that we have a principal (Megillah 2b) - אלה המצות - שאין נביא רשאי לחדש דבר מעתה. A Navi has no write to introduce a new mitvzah, therefore if a Navi seems to be introducing a new mitzvah, that is the greatest proof that it is NOT a NEW mitzvah, rather it was halacha l'moshe mi'sinai until that point. Based on this it makes sense that a kohein who serves in the mikdash pa'ruah rosh will be chayev misah, even though it is only learned from a pasuk in Yechezkiel. We also learn from Yechzekiel that bigdei kehuna need to be linen (Yoma 71b) and that one who is chayuv 2 misos beis din gets the harsher one (sanhedrin 81a). All these halachos are d'oraysa.
Paranthetically, the Ramban understands that אלה המצות sets the limitation of a navi, not only regarding when we are not to believe him, but even regarding the status of the navi himself. The Ramban in Parshas Va'eschanan writes that a Navi who introduces a new mitzvah is not only in violation of אלה המצות, but we learn from there that since he cannot introduce a new mitzvah he will automatically be in violation of bal tosif (like anyone else who adds a new mitzvah). Also, the Ramban in Parshas Re'eh writes that a Navi who attempts to introduce a new mitzvah, would automatically render himself a navi sheker since we are told that he doesn't have that ability, and therefore he would be chayev misah.