perfcounters/powerpc: Add support for POWER5 processors

This adds the back-end for the PMU on the POWER5 processor.  This knows
how to use the fixed-function PMC5 and PMC6 (instructions completed and
run cycles).  Unlike POWER6, PMC5/6 obey the freeze conditions and can
generate interrupts, so their use doesn't impose any extra restrictions.

POWER5+ is different and is not supported by this patch.

Signed-off-by: Paul Mackerras <paulus@samba.org>