MFC r317191, r317195: Don't ignore "disabled" CPUs

r317191 (cognet):
ePAPR states that any non-boot CPU will come in "disabled" state. So we
should not consider a "disabled" cpu as a CPU we have to ignore, and we
should use them if they provide a "enable-method".
While I'm there, support "ok" as well as "okay", while ePAPR only accepts
"okay", linux accepts "ok" too so we can expect it to be used.

r317195 (cognet):
Update comment ot reflect realilty, we know also take care of CPUs that
provide a enable-method.