xen/passthrough: iommu: Basic support of device tree assignment

xen/passthrough: iommu: Basic support of device tree assignment

Add IOMMU helpers to support device tree assignment/deassignment. This patch
introduces 2 new fields in the dt_device_node:
    - is_protected: Does the device is protected by an IOMMU
    - domain_list: Pointer to the next device assigned to the same
    domain

This commit contains only support to protected a device with DOM0.
Device passthrough to another guest won't work out-of-box.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Xiantao Zhang <xiantao.zhang@intel.com>