EBB (Event Based Branches) are currently only available on POWER8, so we
should skip them on other CPUs.

I've found that at least one test loops forever on 970MP (cycles_with_freeze_test).
Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
[mpe: Minor change log editing, add skip to cpu_event_vs_ebb_test]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>