net: mscc: ocelot: make use of SerDes PHYs for handling their configuration

Previously, the SerDes muxing was hardcoded to a given mode in the MAC
controller driver. Now, the SerDes muxing is configured within the
Device Tree and is enforced in the MAC controller driver so we can have
a lot of different SerDes configurations.

Make use of the SerDes PHYs in the MAC controller to set up the SerDes
according to the SerDes<->switch port mapping and the communication mode
with the Ethernet PHY.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>