iommu/vt-d: Factor out dmar_alloc_dev_scope() for later reuse

Factor out function dmar_alloc_dev_scope() from dmar_parse_dev_scope()
for later reuse.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>