net: mvneta: support big endian

Use the "swap descriptor" feature of the hardware to properly swap the
descriptors when running in big endian mode. Since the swapping occurs
on 64 bits words, we also need to provide a separate structure layout
for the DMA descriptors between little endian and big endian mode,
like is done in the mv643xx_eth driver.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>