ARM: dts: BCM5301X: Fix MDIO mux binding

This fixes following error for all BCM5301X dts files:
mdio-bus-mux@18003000: compatible: ['mdio-mux-mmioreg'] is too short
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

arch/arm/boot/dts/bcm5301x.dtsi

@@ -370,7 +370,7 @@ mdio: mdio@18003000 {
 	};
 
 	mdio-mux@18003000 {
-		compatible = "mdio-mux-mmioreg";
+		compatible = "mdio-mux-mmioreg", "mdio-mux";
 		mdio-parent-bus = <&mdio>;
 		#address-cells = <1>;
 		#size-cells = <0>;