ppc: move ppc_cpu_lookup_alias() before its first user

ppc: move ppc_cpu_lookup_alias() before its first user

next commit will drop ppc_cpu_lookup_alias() declaration from header
and make it static which will break its last user ppc_cpu_class_by_name()
since ppc_cpu_class_by_name() defined before ppc_cpu_lookup_alias().

To avoid this move ppc_cpu_lookup_alias() right before
ppc_cpu_class_by_name().

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>