The scripts/dtc/checks.c requires that the node have empty "dma-ranges"
property must have the same "#address-cells" and "#size-cells" values as
the parent node. Otherwise, the following warnings is reported:

arch/arm64/boot/dts/qcom/ipq6018.dtsi:185.3-14: Warning \
(dma_ranges_format): /soc:dma-ranges: empty "dma-ranges" property but \
its #address-cells (1) differs from / (2)
arch/arm64/boot/dts/qcom/ipq6018.dtsi:185.3-14: Warning \
(dma_ranges_format): /soc:dma-ranges: empty "dma-ranges" property but \
its #size-cells (1) differs from / (2)

Arnd Bergmann figured out why it's necessary:
Also note that the #address-cells=<1> means that any device under
this bus is assumed to only support 32-bit addressing, and DMA will
have to go through a slow swiotlb in the absence of an IOMMU.
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20201016090833.1892-3-thunder.leizhen@huawei.comSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>