powerpc/32s: Fix cleanup_cpu_mmu_context() compile bug

powerpc/32s: Fix cleanup_cpu_mmu_context() compile bug

Currently pmac32_defconfig with SMP=y doesn't build:

  arch/powerpc/platforms/powermac/smp.c:
  error: implicit declaration of function 'cleanup_cpu_mmu_context'

It would be nice for consistency if all platforms clear mm_cpumask and
flush TLBs on unplug, but the TLB invalidation bug described in commit
01b0f0eae081 ("powerpc/64s: Trim offlined CPUs from mm_cpumasks") only
applies to 64s and for now we only have the TLB flush code for that
platform.

So just add an empty version for 32-bit Book3S.

Fixes: 01b0f0eae081 ("powerpc/64s: Trim offlined CPUs from mm_cpumasks")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Change log based on comments from Nick]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>