KVM: PPC: Remove PPC970 from KVM_BOOK3S_64_HV text in Kconfig

Since the PPC970 support has been removed from the kvm-hv kernel
module recently, we should also reflect this change in the help
text of the corresponding Kconfig option.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexander Graf <agraf@suse.de>