ASoC: generic-dmaengine-pcm: Use SNDRV_DMA_TYPE_DEV_IRAM as default

When allocating memory space for DMA buffer, use on-chip internal SRAM
as default choice to save power. Since the core would allocate memory
from traditional external memory if iram allocation failed, we don't
need to worry about any side effect.

Signed-off-by: Nicolin Chen <b42378@freescale.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>