[POWERPC] spufs: Refuse to load the module when not running on cell

As noticed by David Woodhouse, it's currently possible to mount
spufs on any machine, which means that it actually will get
mounted by fedora.
This refuses to load the module on platforms that have no
support for SPUs.

Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>