powerpc: Fix building of arch/powerpc/mm/mem.o when MEMORY_HOTPLUG=y and SPARSEMEM=n

Currently the kernel fails to build with the above config options with:
  CC      arch/powerpc/mm/mem.o
arch/powerpc/mm/mem.c: In function 'arch_add_memory':
arch/powerpc/mm/mem.c:130: error: implicit declaration of function 'create_section_mapping'

This explicitly includes asm/sparsemem.h in arch/powerpc/mm/mem.c and
moves the guards in include/asm-powerpc/sparsemem.h to protect the
SPARSEMEM specific portions only.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>