[POWERPC] spufs: add sputrace marker parameter names

The markers facility defines the marker parameters to be of the form
'name %format'. Add parameter names to sputrace, to specify the context
and %spu paramerters,  instead of just specifying the '%format' part.

Signed-off-by: Julio M. Merino Vidal <jmerino@ac.upc.edu>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>