I guess that depends what you mean by platform.
The 855 platform is technically called the i855 family, and the 915 platform is technically called the i915 Express family. They don't have any other legitimate names.
Centrino originally only meant the combination of a Mobile 855 chipset, Banias CPU and Intel 2100 Wi-Fi (later including Dothan and 2200 Wi-Fi). I think the only answer to your question is Centrino if that's what you mean by platform. That's all it was called.
Sonoma only means the new Centrino, which is a Mobile 915 chipset, Dothan CPU and Intel 2915 Wi-Fi.
So "Sonoma" isn't really the name for the 915 platform. The Mobile 915 chipset is part of Sonoma, which is the new Centrino, but the actual name of the 915 chipset is just the i915 Express chipset. There really isn't a name for the pre-Sonoma Centrino. It's just Centrino.
All the Intel products have their little codenames that people like to throw around like Banias, Dothan, Odem (855 chipset), Alviso (915 chipset), etc, but there really aren't any codenames for the old Centrino, because it's not even a product. It's just their marketing term.
I've seen 855-based Centrino referred to as "Banias Platform" and 915-based Centrino referred to as "Sonoma Platform," but that's not anything official.