target-ppc: Introduce abstract CPU family types

Instead of assigning *_<family> constants, set .parent to a family type.

Introduce a POWERPC_FAMILY() macro to keep type registration close to
its implementation. This macro will need tweaking later.
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NAlexander Graf <agraf@suse.de>