[POWERPC] cell: PPU Oprofile cleanup patch

This is a clean up patch that includes the following changes:

 -Some comments were added to clarify the code based on feedback
  from the community.
 -The write_pm_cntrl() and set_count_mode() were passed a
  structure element from a global variable.  The argument was
  removed so the functions now just operate on the global directly.
 -The set_pm_event() function call in the cell_virtual_cntr()
  routine was moved to a for-loop before the for_each_cpu loop

Signed-off-by: Carl Love <carll@us.ibm.com>
Signed-off-by: Maynard Johnson <mpjohn@us.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>