phy: cp110-comphy: 2.5G SGMII mode

This patch allow the CP110 comphy to configure some lanes in the
2.5G SGMII mode. This mode is quite close to SGMII 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>