mips: Select SPARSEMEM_EXTREME

Commit c4617318

 ("MIPS: Add NUMA support for Loongson-3") has increased
.bss size of the Octeon kernel from 16k to 16M. Providing the conditions
for SPARSEMEM_EXTREME avoids the waste of memory.

Thomas has tested the loogsoon64 kernel, where .bss is being reduced by
this patch from 16.5M to 515k.

Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>