iommu/ipmmu-vmsa: Enable multi context support

Add support for up to 8 contexts. Each context is mapped to one
domain. One domain is assigned one or more slave devices. Contexts
are allocated dynamically and slave devices are grouped together
based on which IPMMU device they are connected to. This makes slave
devices tied to the same IPMMU device share the same IOVA space.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>