powerpc/32: Include setup.h header file to fix warnings

Make sure to include setup.h to provide the following prototypes:

  - irqstack_early_init
  - setup_power_save
  - initialize_cache_info

Fix the following warnings (treated as error in W=1):

  arch/powerpc/kernel/setup_32.c:198:13: error: no previous prototype for ‘irqstack_early_init’
  arch/powerpc/kernel/setup_32.c:238:13: error: no previous prototype for ‘setup_power_save’
  arch/powerpc/kernel/setup_32.c:253:13: error: no previous prototype for ‘initialize_cache_info’

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>