powerpc/perf: Power8 PMU support

This patch adds support for the power8 PMU to perf.

Work is ongoing to add generic cache events.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>