of: Allow the iommu-map property to omit untranslated devices

In PCI root complex nodes, the iommu-map property describes the IOMMU that
translates each endpoint. On some platforms, the IOMMU itself is presented
as a PCI endpoint (e.g. AMD IOMMU and virtio-iommu). This isn't supported
by the current OF driver, which expects all endpoints to have an IOMMU.
Allow the iommu-map property to have gaps.

Relaxing of_map_rid() also allows the msi-map property to have gaps, which
is invalid since MSIs always reach an MSI controller. In that case
pci_msi_setup_msi_irqs() will return an error when attempting to find the
device's MSI domain.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>