dt-bindings: Add xen,grant-dma IOMMU description for xen-grant DMA ops

The main purpose of this binding is to communicate Xen specific
information using generic IOMMU device tree bindings (which is
a good fit here) rather than introducing a custom property.

Introduce Xen specific IOMMU for the virtualized device (e.g. virtio)
to be used by Xen grant DMA-mapping layer in the subsequent commit.

The reference to Xen specific IOMMU node using "iommus" property
indicates that Xen grant mappings need to be enabled for the device,
and it specifies the ID of the domain where the corresponding backend
resides. The domid (domain ID) is used as an argument to the Xen grant
mapping APIs.

This is needed for the option to restrict memory access using Xen grant
mappings to work which primary goal is to enable using virtio devices
in Xen guests.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1654197833-25362-6-git-send-email-olekstysh@gmail.com
Signed-off-by: Juergen Gross <jgross@suse.com>

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