fix the non-coherent coldfire dma_alloc_coherent v2

[Christoph Hellwig]

Hi all,

this is the next attempt to not return memory that is not DMA coherent
on coldfire/m68knommu.  The last one needed more fixups in the fec
driver, which this versions includes.  On top of that I've also added
a few more cleanups to the core DMA allocation code.

Jim: any work to support the set_uncached and remap method for arm32
should probably be based on this, and patch 3 should make that
selection a little easier.

Changes since v1:
 - sort out the dependency mess in RISCV
 - don't even built non-coherent DMA support for coldfire cores without
   data caches
 - apply the fec workarounds to all coldfire platforms with data caches
 - add a trivial cleanup for m68k dma.c