test(pmu): check if PMUv3 is functional

The PMU is tested for secure world leakage but there are no checks
whether it works in the first place.

The counter and event counters are exercised separately. This is because
the functionality of one does not imply the functionality of the other
(EL3 has separate controls for both). This additionally catches a corner
case with FEAT_HPMN0 missing without failing all tests.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I966d3155cdd6edfde01af32f7c50c3bb3644274a