iommu/ipmmu-vmsa: Add optional root device feature

Add root device handling to the IPMMU driver by allowing certain
DT compat strings to enable has_cache_leaf_nodes that in turn will
support both root devices with interrupts and leaf devices that
face the actual IPMMU consumer devices.

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