net: fec_mxc: allow use with cache enabled

Ensure that transmit and receive buffers are cache-line aligned.
Invalidate cache for each packet as received, update receive buffer
descriptors one cache line at a time, flush cache before transmitting.

Original patch by Marek:
 http://lists.denx.de/pipermail/u-boot/2012-February/117695.html



Signed-off-by: Eric Nelson <eric.nelson at boundarydevices.com>
Acked-by: Marek Vasut <marex@denx.de>
Tested-by: Marek Vasut <marex@denx.de>