MIPS: Alchemy: fix dbdma2

Various fixes for the Au1200/Au1550/Au1300 DBDMA2 code:

- skip cache invalidation if chip has working coherency circuitry.
- invalidate KSEG0-portion of the (physical) data address.
- force the dma channel doorbell write out to bus immediately with
  a sync.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>