iommu: Make .release_device optional

Many drivers do nothing meaningful for .release_device, and it's neatly
abstracted to just two callsites in the core code, so let's make it
optional to implement.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/bda9d3eb4527eac8f6544a15067e2529cca54a2e.1655822151.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>