PPC: Bump MPIC up to 32 supported CPUs

The MPIC emulation is now capable of handling up to 32 CPUs. Reflect that in
the code exporting the numbers out and fix an integer overflow while at it.

Signed-off-by: Alexander Graf <agraf@suse.de>

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v1 -> v2:

  - Max cpus is 15 due to cINT routing
  - Report nb_cpus not MAX_CPUS in MPIC capabilities
