dt-bindings: interconnect: Add a dma interconnect name

The current DT bindings assume that the DMA will be performed by the
devices through their parent DT node, and rely on that assumption for the
address translation using dma-ranges.

However, some SoCs have devices that will perform DMA through another bus,
with separate address translation rules. We therefore need to express that
relationship, through the special interconnect name "dma-mem".

Acked-by: Georgi Djakov <georgi.djakov@linaro.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>