dt-bindings: iommu: dart: Add apple,t6020-dart compatible

dt-bindings: iommu: dart: Add apple,t6020-dart compatible

t6020-dart is compatible to t8110-dart and annotated as such in Apple's
device tree. Add its per-SoC compatible in case t6020 specific handling
becomes necessary. The dart instances on M2 Pro/Max/Ultra based SoCs
differ in one aspect from the M2 based ones. They indicate an IAS of 42
requiring implementing support for a fouth page table level in the dart
driver and its io-pgtable [1].

[1]: https://lore.kernel.org/asahi/20250821-apple-dart-4levels-v2-0-e39af79daa37@jannau.net/

Reviewed-by: Neal Gompa <neal@gompa.dev>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Janne Grunau <j@jannau.net>