Add support for probing PMCGs from DT, which in itself is trivially
simple. Since DT-based systems with SMMUv3 are most likely to be using
Arm Ltd. implementations at the moment, a further treat is that DT lets
us unambiguously detect those such that we can retroactively make them
a standardised IIDR value to present as a userspace identifier.

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