MIPS: Extend DMA_MAYBE_COHERENT logic to DMA_NONCOHERENT use

Setting DMA_MAYBE_COHERENT gives a platform the opportunity to select
use of cache ops at boot.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/6575/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --cc arch/mips/include/asm/io.h
Simple merge

diff --cc arch/mips/mm/c-r4k.c
Simple merge

diff --cc arch/mips/mm/cache.c
Simple merge