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>