[POWERPC] cell: fix building without spufs

It may be desireable to build a kernel for cell without
spufs, e.g. as the initial kboot kernel. This requires
that the SPU specific parts of the core dump and the xmon
code depend on CONFIG_SPU_BASE instead of CONFIG_PPC_CELL.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>