This extends the flashdev API to support the additional calls required
by NAND device backends. This also adds region-relative IOCTLs to
prevent reimplementation of offset logic in top-end flashdev/filesystem
adapters.
This adds an API call allowing for unregistration to include destruction
of private data that is not managed by the flashdev framework. This also
updates the existing consumers of flashdev to use the new private data
destructor callback.
