move PCI::From* methods into CacheSetHelper class

move PCI::From* methods into CacheSetHelper class

The methods itself deal with the helper a lot, so it makes sense to move
them to the helper itself, which helps also if we want to override some
of these methods, the FromString mentioned in the bugreport being the
obvious example.

VCI is spared from this change for now as while it would fit with the
same reasoning it much heavier entangled with the previous
CacheSetHelper change, so moving it now would mean breaking the API.
The PCI change is worthwhile on its own though as it is used by VCI.

Closes: 686221