powerpc/perf_counter: Add support for PPC970 family

This adds the back-end for the PMU on the PPC970 family.

The PPC970 allows events from the ISU to be selected in two different
ways.  Rather than use alternative event codes to express this, we
instead use a single encoding for ISU events and express the
resulting constraint (that you can't select events from all three
of FPU/IFU/VPU, ISU and IDU/STS at the same time, since they all come
in through only 2 multiplexers) using a NAND constraint field, and
work out which multiplexer is used for ISU events at compute_mmcr
time.

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