ASoC: Rename the PSC functions to DMA

Rename the functions in the mpc5200 DMA file from i2s based names to dma
ones to reflect the file they are in.

Signed-off-by: Jon Smirl <jonsmirl@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>