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>
diff --git a/drivers/dma/of-dma.c b/drivers/dma/of-dma.c
index ca31f1b..cbd4a8a 100644
--- a/drivers/dma/of-dma.c
+++ b/drivers/dma/of-dma.c
@@ -194,6 +194,7 @@
 
 	return ERR_PTR(ret_no_channel);
 }
+EXPORT_SYMBOL_GPL(of_dma_request_slave_channel);
 
 /**
  * of_dma_simple_xlate - Simple DMA engine translation function