iommu/arm-smmu-v3: Improve uAPI comment for IOMMU_HW_INFO_TYPE_ARM_SMMUV3

Be specific about what fields should be accessed in the idr result and
give other guidance to the VMM on how it should generate the
vIDR. Discussion on the list, and review of the qemu implementation
understood this needs to be clearer and more detailed.

Link: https://patch.msgid.link/r/0-v1-191e5e24cec3+3b0-iommufd_smmuv3_hwinf_jgg@nvidia.com


Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>