iommu: Implement mapping IOMMUs for PCI devices

Systems such as Apple's M1 and M2 SoCs may have separate IOMMUs
for each PCIe root port.  In this case the right IOMMU for a
PCI device behind a particular root port is described by an
"iommu-map" property in the device tree.  Parse this property
and use it to find the right IOMMU device for PCI devices.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>