pseries: Restructure class_options functions

Currently each of the *_class_options() functions for the pseries-2.1 ..
pseries-2.5 machine types are standalone.  This will become harder to
maintain as new versions are added.

This patch restructures them similarly to x86 where each function calls
the one from the next version, then overrides anything necessary for
compatibility with the specific version and older.

The default behaviour - that for the most recent machine are set up in
the base class initializer spapr_machine_class_init().  Previously it had
some things set up to default to older behaviour with the more recent
machines overriding it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
