[S390] sparsemem: use SPARSEMEM_STATIC if !64BIT.

In case of !64BIT kernel we end up with a zero sized mem_section array.
This happens because NR_MEM_SECTIONS is smaller than SECTIONS_PER_ROOT
but we have:

#define NR_SECTION_ROOTS (NR_MEM_SECTIONS / SECTIONS_PER_ROOT)

and

struct mem_section *mem_section[NR_SECTION_ROOTS];

So fix this by selecting SPARSEMEM_STATIC which makes sure
that SECTIONS_PER_ROOT is 1.

Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>