target-ppc: Add POWER5+ v2.1 CPU model

Let's avoid -cpu host barfing at this PVR.
Linux recognizes it as "POWER5+ (gs) v2.1".

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Message-id: 1375321323-29954-5-git-send-email-afaerber@suse.de
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>