[POWERPC] cell: set ARCH_SPARSEMEM_DEFAULT in Kconfig

The current cell processor support needs sparsemem, so set it as
the default memory model.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Acked-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>