powerpc/perf: Add power8 EBB support

Add logic to the power8 PMU code to support EBB. Future processors would
also be expected to implement similar constraints. At that time we could
possibly factor these out into common code.

Finally mark the power8 PMU as supporting EBB, which is the actual
enable switch which allows EBBs to be configured.

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