MIPS: Alchemy: Default to noncoherent IO on Au1200 AB

CONFIG_DMA_COHERENT is no longer set; default to noncoherent io on
Au1200 revision AB to make USB work.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/6745/


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