powerpc: Move local setup.h declarations to arch includes

Move the few declarations from arch/powerpc/kernel/setup.h
into arch/powerpc/include/asm/setup.h.  This resolves a
sparse warning for arch/powerpc/mm/numa.c which defines
do_init_bootmem() but can't include the setup.h header
in the prior path.

Resolves:
arch/powerpc/mm/numa.c:998:13:
        warning: symbol 'do_init_bootmem' was not declared.
                 Should it be static?

Signed-off-by: Robert C Jennings <rcj@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>