ASoC: fsl-ssi: Use generic DMA bindings if possible

There may be some platforms using fsl-ssi that do not have a DMA driver
with generic DMA bindings. So this patch adds support for the generic
DMA bindings, while still accepting the old "fsl,dma-events" property if
"dmas" is not found.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>