[CELL] spufs: Make signal-notification files readonly for NOSCHED contexts

Reading from the signal{1,2} files requires a spu_acquire_saved, so
make these files write-only for contexts created with
SPU_CREATE_NOSCHED.

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