dt-bindings: net: add DT bindings for Microsemi MIIM

DT bindings for the Microsemi MII Management Controller found on Microsemi
SoCs
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/mscc-miim.txt

+Microsemi MII Management Controller (MIIM) / MDIO
+=================================================
+
+Properties:
+- compatible: must be "mscc,ocelot-miim"
+- reg: The base address of the MDIO bus controller register bank. Optionally, a
+  second register bank can be defined if there is an associated reset register
+  for internal PHYs
+- #address-cells: Must be <1>.
+- #size-cells: Must be <0>.  MDIO addresses have no size component.
+- interrupts: interrupt specifier (refer to the interrupt binding)
+
+Typically an MDIO bus might have several children.
+
+Example:
+	mdio@107009c {
+		#address-cells = <1>;
+		#size-cells = <0>;
+		compatible = "mscc,ocelot-miim";
+		reg = <0x107009c 0x36>, <0x10700f0 0x8>;
+		interrupts = <14>;
+
+		phy0: ethernet-phy@0 {
+			reg = <0>;
+		};
+	};