dt-bindings: iommu: dart: Support specifying the DMA aperture

Apple DARTs are often connected directly to devices that expect only a
portion of their address space to be used for DMA (for example, because
other ranges are mapped directly to something else).

Follow the "#iommu-cells = <4>" example from iommu.txt and allow
specifying the DMA aperture via additional cells. Since DART is used on
64-bit Apple silicon systems use two cells to specify the start and
length of the aperture. A common aperture starts at 0x100_0000_0000
and spans a 36-bit address space.

This corresponds to the vm-base and vm-size properties on the Apple
device tree side of things.

As not all devices require this and to keep compatibility with existing
device trees allow 1 and 5 as values for #iommu-cells.

Signed-off-by: Janne Grunau <j@jannau.net>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

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