mvebu: neta: align DMA buffers

This makes sure the DMA buffers are properly aligned for the
hardware.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Jon Nettleton <jon@solid-run.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>