powerpc: Add optional smp_ops->prepare_cpu SMP callback

Some platforms (will) need to perform allocations before bringing
a new CPU online. Doing it from smp_ops->setup_cpu is the wrong
thing to do:

 - It has no useful failure path (too late)
 - Calling any allocator will enable interrupts prematurely
   causing problems with large decrementer among others

Instead, add a new callback that is called from __cpu_up (so from
the context trying to online the new CPU) at a point where we
can safely allocate and handle failures.

This will be used by XIVE support.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>