powerpc/spufs: add CONFIG_COREDUMP dependency

The kernel test robot pointed out a slightly different error message
after recent commit 5456ffde ("powerpc/spufs: simplify spufs core
dumping") to spufs for a configuration that never worked:

   powerpc64-linux-ld: arch/powerpc/platforms/cell/spufs/file.o: in function `.spufs_proxydma_info_dump':
>> file.c:(.text+0x4c68): undefined reference to `.dump_emit'
   powerpc64-linux-ld: arch/powerpc/platforms/cell/spufs/file.o: in function `.spufs_dma_info_dump':
   file.c:(.text+0x4d70): undefined reference to `.dump_emit'
   powerpc64-linux-ld: arch/powerpc/platforms/cell/spufs/file.o: in function `.spufs_wbox_info_dump':
   file.c:(.text+0x4df4): undefined reference to `.dump_emit'

Add a Kconfig dependency to prevent this from happening again.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200706132302.3885935-1-arnd@arndb.de