of/address: Introduce of_get_next_dma_parent() helper

Add of_get_next_dma_parent() helper which is similar to
__of_get_dma_parent(), but can be used in iterators and decrements the
ref count on the prior parent.

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