net: mvpp2: Add 2.5GBaseT support

The PPv2 controller is able to support 2.5G speeds, allowing to use
2.5GBASET in conjunction with PHYs that use 2500BASEX as their MII
interface when using this mode.

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>