[POWERPC] spufs: Implement /mbox_info, /ibox_info, and /wbox_info.

This patch implements read only access to

/mbox_info - SPU Write Outbound Mailbox
/ibox_info - SPU Write Outbound Interrupt Mailbox
/wbox_info - SPU Read Inbound Mailbox

These files are used by gdb in order to look into the current mailbox
queues without changing the contents at the same time. They are
not meant for general programming use, since the access requires
a context save and is therefore rather slow.

It would be good to complement this patch with one that adds
write support as well.

Signed-off-by: Dwayne Grant McConnell <decimal@us.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>