dt-bindings: iommu: Add Allwinner H6 IOMMU bindings

The Allwinner H6 has introduced an IOMMU. Let's add a device tree binding
for it.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/f3e1633677a9cf9cf36fe3582f0168fae94c1b3e.1589378833.git-series.maxime@cerno.tech


Signed-off-by: Joerg Roedel <jroedel@suse.de>