dt-bindings: net: phy: add g12a mdio mux documentation

Add documentation for the device tree bindings of the MDIO mux of Amlogic
g12a SoC family
Reviewed-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJerome Brunet <jbrunet@baylibre.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/mdio-mux-meson-g12a.txt

+Properties for the MDIO bus multiplexer/glue of Amlogic G12a SoC family.
+
+This is a special case of a MDIO bus multiplexer. It allows to choose between
+the internal mdio bus leading to the embedded 10/100 PHY or the external
+MDIO bus.
+
+Required properties in addition to the generic multiplexer properties:
+- compatible : amlogic,g12a-mdio-mux
+- reg: physical address and length of the multiplexer/glue registers
+- clocks: list of clock phandle, one for each entry clock-names.
+- clock-names: should contain the following:
+  * "pclk"   : peripheral clock.
+  * "clkin0" : platform crytal
+  * "clkin1" : SoC 50MHz MPLL
+
+Example :
+
+mdio_mux: mdio-multiplexer@4c000 {
+	compatible = "amlogic,g12a-mdio-mux";
+	reg = <0x0 0x4c000 0x0 0xa4>;
+	clocks = <&clkc CLKID_ETH_PHY>,
+		 <&xtal>,
+		 <&clkc CLKID_MPLL_5OM>;
+	clock-names = "pclk", "clkin0", "clkin1";
+	mdio-parent-bus = <&mdio0>;
+	#address-cells = <1>;
+	#size-cells = <0>;
+
+	ext_mdio: mdio@0 {
+		reg = <0>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+	};
+
+	int_mdio: mdio@1 {
+		reg = <1>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+
+		internal_ephy: ethernet-phy@8 {
+			compatible = "ethernet-phy-id0180.3301",
+				     "ethernet-phy-ieee802.3-c22";
+			interrupts = <GIC_SPI 9 IRQ_TYPE_LEVEL_HIGH>;
+			reg = <8>;
+			max-speed = <100>;
+		};
+	};
+};