powerpc/spufs: provide context debug file

Add a ctxt file to spufs that shows spu context information that is used
in scheduling. This info can be used for debugging spufs scheduler
issues, and to isolate between application and spufs problems as it
shows a lot of state such as priorities and dispatch counts.

This file contains internal spufs state and is subject to change at any
time, and therefore no applications should depend on it.  The file is
intended for the use of spufs kernel developers.

Signed-off-by: Luke Browning <lukebrowning@us.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>