powerpc/powernv: Add asynchronous token management for powernv

The OPAL firmware only supports a finite number of in-flight asynchronous
operations. Rather than have each subsystem try to manage its own, use a
central management service to hand out tokens.

More work can be done to improve asynchronous behavior, such as funneling
things through a future OPAL heartbeat handler, but capabilities will be
added as needed.

Augment the existing consumers (i2c and sensors) to use this new API.

MFC after: 4 weeks