xen/iommu: protect iommu_add_dt_device() with dtdevs_lock

xen/iommu: protect iommu_add_dt_device() with dtdevs_lock

Protect iommu_add_dt_device() with dtdevs_lock to prevent concurrent access
to add/remove/assign/deassign.
With addition of dynamic programming feature(follow-up patches in this series),
this function can be concurrently accessed by dynamic node add/remove using
device tree overlays.

Signed-off-by: Vikram Garhwal <vikram.garhwal@amd.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>