net: phy: marvell10g: Use 2500BASEX when using 2.5GBASET

The Marvell Alaska family of PHYs supports 2.5GBaseT and 5GBaseT modes,
as defined in the 802.3bz specification.

Upon establishing a 2.5GBASET link, the PHY will reconfigure it's MII
interface to 2500BASEX.

At 5G, the PHY will reconfigure it's interface to 5GBASE-R, but this
mode isn't supported by any MAC for now.

This was tested with :
 - The 88X3310, which is on the MacchiatoBin
 - The 88E2010, an Alaska PHY that has no fiber interfaces, and is
   limited to 5G maximum speed.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>