Introduce functions to support dynamic IOMMU seq_id allocating and
releasing, which will be used to support DMAR hotplug.

Also rename IOMMU_UNITS_SUPPORTED as DMAR_UNITS_SUPPORTED.
Signed-off-by: NJiang Liu <jiang.liu@linux.intel.com>
Reviewed-by: NYijing Wang <wangyijing@huawei.com>
Signed-off-by: NJoerg Roedel <jroedel@suse.de>