ppc: make cpu alias point only to real cpu models

ppc: make cpu alias point only to real cpu models

alias pointing to another alias forces lookup code to
do recurrsive translation till real cpu model is reached.

Drop this nonsence and make each alias point to cpu model
that has corresponding CPU type. It will allow to drop
recurrsion in cpu model translation code and actually
make ppc_cpu_aliases[] content use PowerPCCPUAlias
fields properly
(i.e. alias goes into .alias and model goes into .model)

While at it add TODO defines around aliases that point to
cpu models excluded by the same TODO defines.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>