ASoC: ux500: Add DMA slave config prepare routine

Implement a DMA slave config prepare routine, as until now the MSP
driver depended on the DMA controller completing the channel
configuration on its own, but this is not the case anymore since the
recent DMA driver updates.

Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>