xen/arm: domain_build: Don't expose IOMMU specific properties to hwdom

xen/arm: domain_build: Don't expose IOMMU specific properties to hwdom

We always skip the IOMMU device when creating DT for hwdom if there is
an appropriate driver for it in Xen (device_get_class(iommu_node)
returns DEVICE_IOMMU). So, even if it is not used by Xen it will be skipped.

We should also skip the IOMMU specific properties of the master device
behind that IOMMU in order to avoid exposing an half complete IOMMU
bindings to hwdom.

According to the Linux's docs:
1. Documentation/devicetree/bindings/iommu/iommu.txt
2. Documentation/devicetree/bindings/pci/pci-iommu.txt

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Release-acked-by: Juergen Gross <jgross@suse.com>