powerpc/topology: Get topology for shared processors at boot

On a shared LPAR, Phyp will not update the CPU associativity at boot
time. Just after the boot system does recognize itself as a shared
LPAR and trigger a request for correct CPU associativity. But by then
the scheduler would have already created/destroyed its sched domains.

This causes
  - Broken load balance across Nodes causing islands of cores.
  - Performance degradation esp if the system is lightly loaded
  - dmesg to wrongly report all CPUs to be in Node 0.
  - Messages in dmesg saying borken topology.
  - With commit 051f3ca0 ("sched/topology: Introduce NUMA identity
    node sched domain"), can cause rcu stalls at boot up.

The sched_domains_numa_masks table which is used to generate cpumasks
is only created at boot time just before creating sched domains and
never updated. Hence, its better to get the topology correct before
the sched domains are created.

For example on 64 core Power 8 shared LPAR, dmesg reports

  Brought up 512...