[POWERPC] spufs: add mode= mount option

Add a 'mode=' option to spufs mount arguments. This allows more
control over access to the top-level spufs directory.

Tested on Cell.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>