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>