sparsemem: make SPARSEMEM_VMEMMAP selectable

SPARSEMEM_VMEMMAP needs to be a selectable config option to support
building the kernel both with and without sparsemem vmemmap support.  This
selection is desirable for platforms which could be configured one way for
platform specific builds and the other for multi-platform builds.

Signed-off-by: Miguel Botón <mboton@gmail.com>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Acked-by: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>