dt-bindings: dmaengine: sprd: Change channel id to slave id for DMA cell specifier

For Spreadtrum DMA engine, all channels are equal, which means slave can
request any channels with setting a unique slave id to trigger this channel.

Thus we can remove the channel id from device tree to assign the channel
dynamically, moreover we should add the slave id in device tree.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>