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>

arch/mips/boot/dts/mscc/ocelot.dtsi

@@ -234,6 +234,11 @@ mdio1: mdio@10700c0 {
 		hsio: syscon@10d0000 {
 			compatible = "mscc,ocelot-hsio", "syscon", "simple-mfd";
 			reg = <0x10d0000 0x10000>;
+
+			serdes: serdes {
+				compatible = "mscc,vsc7514-serdes";
+				#phy-cells = <2>;
+			};
 		};
 	};
 };