smp: Start up non-boot CPUs asynchronously

[Benjamin Herrenschmidt]

On Tue, 2012-02-14 at 15:18 +0530, Srivatsa S. Bhat wrote:

> > 2. Just below that we have smp_cpus_done(setup_max_cpus); and this translates
> > to native_smp_cpus_done() under x86, which calls impress_friends().
> > And that means, the bogosum calculation and the total activated processor
> > count which is printed, may get messed up.

We also have code on powerpc that relies on the bringup having been
completed in smp_cpus_done(), especially on platforms that don't support
CPU hotplug (or fake it using sleep loops).

In some case we unmap MMIO space or close access to components (i2c for
example) that we use during the bringup for things like hard synchro of
CPU timebases, etc... on some G5s we disable the elastic interface on
the northbridge for CPUs that weren't brought up, that sort of thing...

So this patch will break a LOT of stuff for us, it must at least be a
config option for now, until we find another way to fix these things.

Cheers,
Ben.