MIPS: BMIPS: Flush the readahead cache after DMA.

BMIPS 3300/435x/438x CPUs have a readahead cache that is separate from
the L1/L2.  During a DMA operation, accesses adjacent to a DMA buffer
may cause parts of the DMA buffer to be prefetched into the RAC.  To
avoid possible coherency problems, flush the RAC upon DMA completion.

Derived from Kevin Cernekee's https://patchwork.linux-mips.org/patch/9602/

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