perfcounters/powerpc: add support for POWER5+ processors

Impact: more hardware support

This adds the back-end for the PMU on the POWER5+ processors (i.e. GS,
including GS DD3 aka POWER5++).  This doesn't use the fixed-function
PMC5 and PMC6 since they don't respect the freeze conditions and don't
generate interrupts, as on POWER6.

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