MFC: r219548

MFC: r219548

Allocate the sound buffer DMA memory coherent. While NetBSD typically
also does this for sound drivers it's probably not necessary for all
combinations of controllers and drivers. However, given that our sound
drivers completely lack bus_dmamap_sync(9) calls this at least serves
as a workaround when enabling use of the IOMMU streaming buffers on
sparc64 and generally for arm and mips.

git-svn-id: svn://svn.freebsd.org/base/stable/8@219995 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f