MIPS: IP27: Switch from DMA_IP27 to DMA_COHERENT

The special IP27 DMA code selected by DMA_IP27 has been removed a while
ago turning DMA_IP27 into almost a nop.  Also fixup the broken logic of
its last users memcpy.S and memcpy-inatomic.s.

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