arm64: dma-mapping: Fix arm_iommu_get_dma_window

Since of_dma_get_range searches the parent device node, it cannot
be used to describe the iommu address range desired by a particular
device.

Current users will be converted in a separate patch, as they
are broken anyway.

Change-Id: I2e203e051120c840c12802b1952170b6a1cd7967
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>