[POWERPC] cell: add low-level performance monitoring code

Add routines for accessing the registers and counters in the performance
monitoring unit.

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