[MIPS] Unify dma-{coherent,noncoherent.ip27,ip32}

Platforms will now have to supply a function dma_device_is_coherent which
returns if a particular device participates in the coherence domain.  For
most platforms this function will always return 0 or 1.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>