iommu/ipmmu-vmsa: Use fwspec iommu_priv on ARM64

Convert from archdata to iommu_priv via iommu_fwspec on ARM64 but
let 32-bit ARM keep on using archdata for now.

Once the 32-bit ARM code and the IPMMU driver is able to move over
to CONFIG_IOMMU_DMA=y then coverting to fwspec via ->of_xlate() will
be easy.

For now fwspec ids and num_ids are not used to allow code sharing between
32-bit and 64-bit ARM code inside the driver.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Joerg Roedel <jroedel@suse.de>