The mvneta controller can handle speeds up to 2500Mbps on the SGMII
interface. This relies on serdes configuration, the lane must be
configured at 3.125Gbps and we can't use in-band autoneg at that speed.

The main issue when supporting that speed on this particular controller
is that the link partner can send ethernet frames with a shortened
preamble, which if not explicitly enabled in the controller will cause
unexpected behaviours.

This was tested on Armada 385, with the comphy configuration done in
bootloader.
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>