perf: Add ARM SMMU PMU driver

Add a basic driver for the SMMUv2 Performance Monitors Extension. This
exposes the architecturally-defined events along with a fairly low-level
interface, based on the relevant register fields, for filtering.

The relationship between Stream ID Groups and Counter Groups, i.e. which
subsets of events may be visible to which sets of counters, is entirely
implementation defined and non-discoverable, and attempting to define
firmware bindings to describe the mappings would be considerable work
for very little benefit. Thus we expect the user to have the relevant
implementation knowledge, and explicitly specify the appropriate counter
group for each desired event.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>