netdev/phy: add MDIO bus multiplexer driven by a memory-mapped device

Add support for an MDIO bus multiplexer controlled by a simple memory-mapped
device, like an FPGA.  The device must be memory-mapped and contain only
8-bit registers (which keeps things simple).

Tested on a Freescale P5020DS board which uses the "PIXIS" FPGA attached
to the localbus.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/mdio-mux-mmioreg.txt

@@ -0,0 +1,75 @@
+Properties for an MDIO bus multiplexer controlled by a memory-mapped device
+
+This is a special case of a MDIO bus multiplexer.  A memory-mapped device,
+like an FPGA, is used to control which child bus is connected.  The mdio-mux
+node must be a child of the memory-mapped device.  The driver currently only
+supports devices with eight-bit registers.
+
+Required properties in addition to the generic multiplexer properties:
+
+- compatible : string, must contain "mdio-mux-mmioreg"
+
+- reg : integer, contains the offset of the register that controls the bus
+	multiplexer.  The size field in the 'reg' property is the size of
+	register, and must therefore be 1.
+
+- mux-mask : integer, contains an eight-bit mask that specifies which
+	bits in the register control the actual bus multiplexer.  The
+	'reg' property of each child mdio-mux node must be constrained by
+	this mask.
+
+Example:
+
+The FPGA node defines a memory-mapped FPGA with a register space of 0x30 bytes.
+For the "EMI2" MDIO bus, register 9 (BRDCFG1) controls the mux on that bus.
+A bitmask of 0x6 means that bits 1 and 2 (bit 0 is lsb) are the bits on
+BRDCFG1 that control the actual mux.
+
+	/* The FPGA node */
+	fpga: board-control@3,0 {
+		#address-cells = <1>;
+		#size-cells = <1>;
+		compatible = "fsl,p5020ds-fpga", "fsl,fpga-ngpixis";
+		reg = <3 0 0x30>;
+		ranges = <0 3 0 0x30>;
+
+		mdio-mux-emi2 {
+			compatible = "mdio-mux-mmioreg", "mdio-mux";
+			mdio-parent-bus = <&xmdio0>;
+			#address-cells = <1>;
+			#size-cells = <0>;
+			reg = <9 1>; // BRDCFG1
+			mux-mask = <0x6>; // EMI2
+
+			emi2_slot1: mdio@0 {	// Slot 1 XAUI (FM2)
+				reg = <0>;
+				#address-cells = <1>;
+				#size-cells = <0>;
+
+				phy_xgmii_slot1: ethernet-phy@0 {
+					compatible = "ethernet-phy-ieee802.3-c45";
+					reg = <4>;
+				};
+			};
+
+			emi2_slot2: mdio@2 {	// Slot 2 XAUI (FM1)
+				reg = <2>;
+				#address-cells = <1>;
+				#size-cells = <0>;
+
+				phy_xgmii_slot2: ethernet-phy@4 {
+					compatible = "ethernet-phy-ieee802.3-c45";
+					reg = <0>;
+				};
+			};
+		};
+	};
+
+	/* The parent MDIO bus. */
+	xmdio0: mdio@f1000 {
+		#address-cells = <1>;
+		#size-cells = <0>;
+		compatible = "fsl,fman-xmdio";
+		reg = <0xf1000 0x1000>;
+		interrupts = <100 1 0 0>;
+	};