[POWERPC] spufs: fix memory hotplug dependency

spufs_base.c calls __add_pages, which depends on CONFIG_MEMORY_HOTPLUG.

Moved the selection of CONFIG_MEMORY_HOTPLUG from CONFIG_SPUFS_MMAP
to CONFIG_SPU_FS.

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