fpi-device: Add FpiDeviceFeature flags to FpDeviceClass
As there was some related work in progress in !256 (merged), I've take back some parts I was hacking as part of !201 (closed), and finally using flags for features.
Allows drivers to define more fine grained features for devices, not strictly depending on assumptions we can make depending on the implemented vfuncs.
We keep this per class but could be in theory moved to each instance.
In any case, added an utility function to initialize it in the way we can ensure that we've a consistent way for setting them across all the devices.
In the next commits I'm also moving the flags to be public and deprecating the old functions, we can strip the commit out if wanted for now.
/cc @vincenth