target-sh4: Introduce SuperHCPU subclasses

Store legacy name in SuperHCPUClass for -cpu ? and for case-insensitive
class lookup.

List CPUs by iterating over TYPE_SUPERH_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>