net: mvpp2: 2500baseX support

This patch adds the 2500Base-X PHY mode support in the Marvell PPv2
driver. 2500Base-X is quite close to 1000Base-X and SGMII modes and uses
nearly the same code path.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>