of: Make of_dma_get_range() private

of_dma_get_range() is only used within the DT core code, so remove the
export and move the header declaration to the private header.

Cc: 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>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rob Herring <robh@kernel.org>