arch: arm64: apple: t600x: Use standard "iommu" node name

The PCIe iommu nodes use "dart" as node names. Replace it with the
the standard "iommu" node name as all other iommu nodes.

Fixes: 7b0b0191 ("arm64: dts: apple: Add initial t6000/t6001/t6002 DTs")
Signed-off-by: Janne Grunau <j@jannau.net>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Signed-off-by: Hector Martin <marcan@marcan.st>