dmaengine: export symbol of of_dma_request_slave_channel()

Current DMAEngine implementation of DT bindings can't support
DT subnode. This patch export symbols of of_dma_request_slave_channel()
for subnode DMA DT bingings.

ex)

rcar_sound: rcar_sound@ec500000 {
        ...
        rcar_sound,dvc {
                dvc0: dvc@0 {
                        dmas = <&audma0 0xbc>;
                        dma-names = "tx";
                };
                dvc1: dvc@1 {
                        dmas = <&audma0 0xbe>;
                        dma-names = "tx";
                };
        };
        ...
};
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/dma/of-dma.c

@@ -194,6 +194,7 @@ struct dma_chan *of_dma_request_slave_channel(struct device_node *np,
 
 	return ERR_PTR(ret_no_channel);
 }
+EXPORT_SYMBOL_GPL(of_dma_request_slave_channel);
 
 /**
  * of_dma_simple_xlate - Simple DMA engine translation function