Hey there, welcome to the boards.
Currently there is only one gate inop because its jetway has been detached and this is A6. This is the only time I've ever seen a jet bridge decoupled from the concourse in PHX since I've been here. I am not sure exactly why it was taken down; my suspicion is repairs since the A6 jetway has had a reputation for going kaput from time to time. Though since it was taken down I have not seen anyone working on it.
I don't think it has to do with the 321's since they were already going into A6 without any kind of obvious problem up to this point. A few gates had their Airbus J-lines extended and another block added for the 321's more recently, these being A3, A6, B6, and A23, and all of these could already accommodate 757's on a separate J-line except A23. Including the international gates, 13 out of 43 mainline-use gates currently have 321 blocks on them.
On the subject, there are a number of gates that are currently 737 only and these may need to be re-engineered/removed/shifted in order to accommodate 319's/320's (since the 737's are being phased out of the West fleet); this unless some E190's make their way out here.
Jetway adjustments would also be conceivably necessary if US were to want a widebody capable gate of our own for domestic service (Hawaii or CLT), i.e. not have to use the shared gates on the international concourse (but any WB service through PHX is a long shot).
Hope that answers your question.
Cheers!