iommu/ipmmu-vmsa: Clean up device tracking

Get rid of now unused device tracking code. Future code should instead
be able to use driver_for_each_device() for this purpose.

This is a simplified version of the following patch from Robin
[PATCH] iommu/ipmmu-vmsa: Clean up group allocation

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Joerg Roedel <jroedel@suse.de>