ASoC: omap-pcm: Move omap-pcm under include/sound

Make including the omap-pcm.h outside sound/soc/omap more convenient.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>