phy: add 2.5G SGMII mode to the phy_mode enum

This patch adds one more generic PHY mode to the phy_mode enum, to allow
configuring generic PHYs to the 2.5G SGMII mode by using the set_mode
callback.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>