Super Bowl locker room assignments

Yes, the Bucs used their own locker rooms; see this quote from Head Coach Bruce Arians:

“It’s really, really amazing to just be in our facility, be in our locker room, on the practice field, same drive to work every single day,” Bucs coach Bruce Arians said. “Being able to stay in that routine has been critical, because normally you’re jumping on a plane.”

Also confirmed this via this article about their celebration:

With the game being held at Raymond James Stadium, the Buccaneers were able to celebrate in the confines of their home locker room.

However, your question is slightly wrong: the Bucs were the home team this year, and so it's moot unclear what would've happened had your question's assumption been accurate. See this article for example:

There's nothing fancy about the home team determination for the Super Bowl. It alternates each year from the AFC representative to the NFC representative and back again.

A year ago, the Chiefs were the home team as they won Super Bowl 54. For Super Bowl 55, that means the Buccaneers are the home team while the Chiefs are the away team. It's purely coincidental that the designations align with the game being played in Tampa Bay's home stadium.

It would've been interesting to see what happened with locker rooms if the Buccaneers were deemed the visitors. Would they have had to let the Chiefs use the home locker room while they dressed in the away locker room? Alas, the timing didn't work out for such a discovery in 2021.