net: fec: use dma_alloc_noncoherent for data cache enabled coldfire
Coldfire platforms with data caches can't properly implement
dma_alloc_coherent and currently just return noncoherent memory from
dma_alloc_coherent.
The fec driver than works around this with a flush of all caches in the
receive path. Make this hack a little less bad by using the explicit
dma_alloc_noncoherent API and documenting the hacky cache flushes so
that the DMA API level hack can be removed.
Also replace the check for CONFIG_M532x for said hack with a check
for COLDFIRE && !COLDFIRE_COHERENT_DMA. While m532x is the only such
platform with a fec module, this makes the code more consistent and
easier to follow.