MIPS/malta: simplify plat_setup_iocoherency

Given that plat_mem_setup runs before earlyparams are handled and malta
selects CONFIG_DMA_MAYBE_COHERENT, coherentio can only be set to
IO_COHERENCE_DEFAULT at this point.  So remove the checking for other
options and merge plat_enable_iocoherency into plat_setup_iocoherency
to simplify the code a bit.

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