iommu/ipmmu-vmsa: Hook up r8a7795 DT matching code

Tie in r8a7795 features and update the IOMMU_OF_DECLARE
compat string to include the updated compat string.

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