net: mvneta: Add support for 2500Mbps SGMII

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>