Upstream support for pmbus fan-controller bindings

[Andrew Jeffery]

Currently we have out-of-tree patches for the max31785 that define devicetree bindings for PMBus fan control. Rob Herring wasn't convinced by the effort, hence the changes have lived out of tree. Try again, though this time clarify that we're defining bindings for a standardised fan-controller interface and not for fans themselves (which was the original sticking point).