perf/smmuv3: Support sysfs identifier file

SMMU_PMCG_IIDR was added in the SMMUv3.3 spec.

For the perf tool to know the specific HW implementation, expose the
PMCG_IIDR contents only when set.

Signed-off-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/1602149181-237415-5-git-send-email-john.garry@huawei.com


Signed-off-by: Will Deacon <will@kernel.org>