ASoC: Fix i.MX audio build for i.MX3x

Don't unconditionally include the i.MX2x DMA driver, the arch/arm
functions it uses aren't available for i.MX3x.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Javier Martin <javier.martin@vista-silicon.com>