powerpc/pseries/iommu: Fix the VFIO_IOMMU_SPAPR_TCE_GET_INFO ioctl output

The ioctl VFIO_IOMMU_SPAPR_TCE_GET_INFO is not reporting the
actuals on the platform as not all the details are correctly
collected during the platform probe/scan into the iommu_table_group.

Collect the information during the device setup time as the DMA
window property is already looked up on parent nodes anyway.

Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/171923271138.1397.7908302630061814623.stgit@linux.ibm.com