MIPS: mscc: ocelot: add SerDes mux DT node

The Microsemi Ocelot has a set of register for SerDes/switch port muxing
as well as PCIe muxing for a specific SerDes, so let's add the device
and all SerDes in the Device Tree.

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