iommu/vt-d: Dynamically allocate and free seq_id for DMAR units

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: Jiang Liu <jiang.liu@linux.intel.com>
Reviewed-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>