KVM: PPC: Add Gekko SPRs

The Gekko has some SPR values that differ from other PPC core values and
also some additional ones.

Let's add support for them in our mfspr/mtspr emulator.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>