dt-bindings: add constants for Microsemi Ocelot SerDes driver

The Microsemi Ocelot has multiple SerDes and requires that the SerDes be
muxed accordingly to the hardware representation.

Let's add a constant for each SerDes available in the Microsemi Ocelot.

Reviewed-by: Rob Herring <robh@kernel.org>
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>