iommu/vt-d: Enhance intel_irq_remapping driver to support DMAR unit hotplug

Implement required callback functions for intel_irq_remapping driver
to support DMAR unit hotplug.

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