New features:
  - Slave Clock Stretch Select,
  - Fast Mode+ Enable,
  - Enhanced First Bit Setup Cycle configuration.

Looks like the R-Car V3U variant can be treated as a generic R-Car Gen3
variant (lacking the new features, of course).

FM+ doesn't need an update. It will just handle higher values for the
bus frequency. This is also the most likely one to be supported in
Linux. First Bit setup is not even supported yet in the basic version.
Configuration via Linux is unclear same as SlaveClockStretch. Use cases
are unclear, too, which we would need for implementating any of this.