[POWERPC] spufs: add support for read/write on cntl

Writing to cntl can be used to stop execution on the
spu and to restart it, reading from cntl gives the
contents of the current status register.

The access is always in ascii, as for most other files.

This was always meant to be there, but we had a little
problem with writing to runctl so it was left out so
far.

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