target-sh4: Introduce SuperHCPU subclasses

Store legacy name in SuperHCPUClass.
List CPUs by iterating over CPU subclasses.

Signed-off-by: Andreas Färber <afaerber@suse.de>
target-sh4: Move PVR/PRR/CVR into SuperHCPUClass

They are never changed once initialized, and moving them to the class
will allow to inspect them before instantiating.

Signed-off-by: Andreas Färber <afaerber@suse.de>