The AMD Seattle DWC AHCI is behind an IOMMU and has 1-3 entries, so add
the 'iommus' property. There's not a specific compatible, so we can't
limit it to Seattle.
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Niklas Cassel <cassel@kernel.org>