selftests/powerpc: Run EBB tests only on POWER8

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>