MIPS/alchemy: factor out the DMA coherent setup

Factor out a alchemy_dma_coherent helper that determines if the platform
is DMA coherent.  Also stop initializing the hw_coherentio variable, given
that is only ever set to a non-zero value by the malta setup code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>