powerpc/mem: Add back missing header to fix 'no previous prototype' error

Commit b26e8f27 ("powerpc/mem: Move cache flushing functions into
mm/cacheflush.c") removed asm/sparsemem.h which is required when
CONFIG_MEMORY_HOTPLUG is selected to get the declaration of
create_section_mapping().

Add it back.

Fixes: b26e8f27 ("powerpc/mem: Move cache flushing functions into mm/cacheflush.c")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3e5b63bb3daab54a1eb9c20221c2e9528c4db9b3.1622883330.git.christophe.leroy@csgroup.eu