net: dsa: sja1105: plug in support for 2500base-x

The MAC treats 2500base-x same as SGMII (yay for that) except that it
must be set to a different speed.

Extend all places that check for SGMII to also check for 2500base-x.

Also add the missing 2500base-x compatibility matrix entry for SJA1110D.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>