powerpc/mpic: Limit NR_CPUS loop to 32 bit

mpic_physmask was looping NR_CPUS times over a mask that was passed as
a u32. Since mpic is architecturaly limited to 32 physical cpus, clamp
the logical cpus to 32 when compiling (we could also clamp at runtime
to nr_cpu_ids).

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>