ALSA: pcm - Use dma_mmap_coherent() if available

Use dma_mmap_coherent() for mmapping the buffers allocated via
dma_alloc_coherent() if available.  Currently, only ARM has this function,
so we do temporarily have an ifdef pcm_native.c.  This should be handled
better globally in future.

Signed-off-by: Takashi Iwai <tiwai@suse.de>