iommu/arm: Add iommu_dt_xlate()

iommu/arm: Add iommu_dt_xlate()

Move code for processing DT IOMMU specifier to a separate helper.
This helper will be re-used for adding PCI devices by the subsequent
patches as we will need exact the same actions for processing
DT PCI-IOMMU specifier.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
Signed-off-by: Mykyta Poturai <mykyta_poturai@epam.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>