of: address: Follow DMA parent for "dma-coherent"

Much like for address translation, when checking for DMA coherence we
should be sure to walk up the DMA hierarchy, rather than the MMIO one,
now that we can accommodate them being different.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Tested-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Rob Herring <robh@kernel.org>