xen/iommu: Consolidate device assignment ops into a single set

xen/iommu: Consolidate device assignment ops into a single set

On ARM, the way to assign device tree node is exactly the same as PCI.
Futhermore, all devices can be represented by a 'device_t'.
Therefore there is no need to add separate ops.

The x86 iommu drivers has not been modified to replace 'struct pci_dev'
by "device_t" because the latter is an alias of the former.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>\
Acked-by: Ian Campbell <ian.campbell@citrix.com>