dt-bindings: Add Arm SMMUv3 PMCG binding

Add binding for the Arm SMMUv3 PMU. Each node represents a PMCG, and is
placed as a sibling node of the SMMU. Although the PMCGs registers may
be within the SMMU MMIO region, they are separate devices, and there can
be multiple PMCG devices for each SMMU (for example one for the TCU and
one for each TBU).

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20211117144844.241072-2-jean-philippe@linaro.org
Signed-off-by: Will Deacon <will@kernel.org>