Device tree structures used by register based MDIO muxes is described here.
This binding is based on reg-mux.txt binding in Linux and is currently used by
mdio-mux-i2creg driver in U-Boot.

Required properties:
#mux-control-cells = <1> indicates how many registers are used for mux
			selection.  mux-reg-mask property described below must
			include this number of pairs.
mux-reg-masks = <reg mask> describes pairs of register offset and register mask.
			Register bits enabled in mask are set to the selection
			value defined in reg property of child MDIOs to control
			selection.
Properties described in mdio-mux.txt also apply.

Example structure, used on Freescale LS1028A QDS board:

&i2c0 {
	status = "okay";
	u-boot,dm-pre-reloc;

	fpga@66 {
		#address-cells = <1>;
		#size-cells = <0>;
		compatible = "simple-mfd";
		reg = <0x66>;

		mux-mdio@54 {
			#address-cells = <1>;
			#size-cells = <0>;
			compatible = "mdio-mux-i2creg";
			reg = <0x54>;
			#mux-control-cells = <1>;
			mux-reg-masks = <0x54 0xf0>;
			mdio-parent-bus = <&mdio0>;

			/* on-board MDIO with a single RGMII PHY */
			mdio@00 {
				#address-cells = <1>;
				#size-cells = <0>;
				reg = <0x00>;

				/* on-board 1G RGMII PHY */
				qds_phy0: phy@5 {
					reg = <5>;
				};
			};
			/* card slot 1 */
			mdio@40 {
				#address-cells = <1>;
				#size-cells = <0>;
				reg = <0x40>;
				/* VSC8234 1G SGMII card */
				sgmii_port0: phy@1c {
					reg = <0x1c>;
				};
			};
			/* card slot 2 */
			mdio@50 {
				#address-cells = <1>;
				#size-cells = <0>;
				reg = <0x50>;
			};
			/* card slot 3 */
			mdio@60 {
				#address-cells = <1>;
				#size-cells = <0>;
				reg = <0x60>;
			};
			/* card slot 4 */
			mdio@70 {
				#address-cells = <1>;
				#size-cells = <0>;
				reg = <0x70>;
			};
		};
	};
};

/* Parent MDIO, defined in SoC .dtsi file, just enabled here */
&mdio0 {
	status = "okay";
};