net: ethernet: mvneta: Add 2500BaseX support for SoCs without comphy

The older SoCs like Armada XP support a 2500BaseX mode in the datasheets
referred to as DR-SGMII (Double rated SGMII) or HS-SGMII (High Speed
SGMII). This is an upclocked 1000BaseX mode, thus
PHY_INTERFACE_MODE_2500BASEX is the appropriate mode define for it.
adding support for it merely means writing the correct magic value into
the MVNETA_SERDES_CFG register.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>