of: address: Add support for the parent DMA bus

Some SoCs have devices that are using a separate bus from the main bus to
perform DMA.

These buses might have some restrictions and/or different mapping than from
the CPU side, so we'd need to express those using the usual dma-ranges, but
using a different DT node than the node's parent.

Now that the generic interconnect bindings are available, we can model an
interconnect with the reserved name "dma-mem" for those use-cases.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>