[POWERPC] OProfile: enable callgraph support for Cell

This patch enables OProfile callgraph support for the Cell processor.  The
original code was just calling a function to add the PC value, now it will
call a function that first checks the callgraph depth.  Callgraph is already
enabled on the other Power platforms.

Signed-off-by: Bob Nelson <rrnelson@us.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>