Buffers must not have an unclean cache before being used for DMA - a
pending write-back may corrupt the next dev-to-mem transfer otherwise.

This was consistently noticeable during long TFTP transfers, when an ARP
request is answered by U-Boot in the middle of the transfer:

As U-Boot's arp_receive() reuses the receive buffer to prepare its
reply packet, the beginning of one of the next incoming TFTP packets
is overwritten by the ARP reply. The corrupted packet is ignored, but
the TFTP transfer stalls for a few seconds until a timeout is detected
and a retransmit is triggered.

Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Tested-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>