iommu/ipmmu-vmsa: Hook up r8a7796 DT matching code

Support the r8a7796 IPMMU by sharing feature flags between
r8a7795 and r8a7796. Also update IOMMU_OF_DECLARE to hook
up the updated compat string.

[rebased on v4.17]
Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Joerg Roedel <jroedel@suse.de>