net: dsa: sja1105: cache the phy-mode port property

So far we've succeeded in operating without keeping a copy of the
phy-mode in the driver, since we already have the static config and we
can look at the xMII Mode Parameters Table which already holds that
information.

But with the SJA1110, we cannot make the distinction between sgmii and
2500base-x, because to the hardware's static config, it's all SGMII.
So add a phy_mode property per port inside struct sja1105_private.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>