[S390] fix sparsemem related compile error with allnoconfig on s390

On s390 make allnoconfig fails with the following build error:

arch/s390/mm/init.c: In function 'show_mem':
arch/s390/mm/init.c:55: error: implicit declaration of function 'pfn_valid'
make[1]: *** [arch/s390/mm/init.o] Error 1
make: *** [arch/s390/mm] Error 2

This problem can by fixed ensuring that ARCH_SELECT_MEMORY_MODEL
is always turned on.

Signed-off-by: Hans-Joachim Picht <hans@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>