ALSA: Add MAINTAINERS entry for dmaengine helpers

Help ensure that Lars-Peter gets CCed on dmaengine related patches by
adding a MAINTAINERS entry for the helpers.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>