[POWERPC] spufs: fix mismerge, making context signal{1,2} files readable again

The commit 8b6f50ef seems to have
been affected by a mismerge of a duplicate patch
(d054b36f) - both the
spufs_dir_contents and spufs_dir_nosched_contents have been given
write-only signal notification files.

This change reverts the spufs_dir_contents array to use the
readable signal notification file implementation.

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