iommu: Add M2 support to Apple DART driver

"apple,t8112-dart" uses an incompatible register interface but still
offers the same functionality. This DART is found on the M2 and M1
Pro/Max/Ultra SoCs.

Signed-off-by: Janne Grunau <j@jannau.net>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>