dt-bindings: iommu: add DART iommu bindings

DART (Device Address Resolution Table) is the iommu found on Apple
ARM SoCs such as the M1.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Signed-off-by: Sven Peter <sven@svenpeter.dev>
Link: https://lore.kernel.org/r/20210803121651.61594-3-sven@svenpeter.dev
Signed-off-by: Joerg Roedel <jroedel@suse.de>