usb: host: ohci-st: Add ohci-st devicetree bindings documentation

This patch documents the device tree bindings required for
the ohci on-chip controller found in ST consumer electronics SoC's.
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/ohci-st.txt

+ST USB OHCI controller
+
+Required properties:
+
+ - compatible		: must be "st,st-ohci-300x"
+ - reg			: physical base addresses of the controller and length of memory mapped
+			  region
+ - interrupts		: one OHCI controller interrupt should be described here
+ - clocks		: phandle list of usb clocks
+ - clock-names		: should be "ic" for interconnect clock and "clk48"
+See: Documentation/devicetree/bindings/clock/clock-bindings.txt
+
+ - phys			: phandle for the PHY device
+ - phy-names		: should be "usb"
+
+ - resets		: phandle to the powerdown and reset controller for the USB IP
+ - reset-names		: should be "power" and "softreset".
+See: Documentation/devicetree/bindings/reset/st,sti-powerdown.txt
+See: Documentation/devicetree/bindings/reset/reset.txt
+
+Example:
+
+	ohci0: usb@0xfe1ffc00 {
+		compatible = "st,st-ohci-300x";
+		reg = <0xfe1ffc00 0x100>;
+		interrupts = <GIC_SPI 149 IRQ_TYPE_NONE>;
+		clocks = <&clk_s_a1_ls 0>,
+			 <&clockgen_b0 0>;
+		clock-names = "ic", "clk48";
+		phys = <&usb2_phy>;
+		phy-names = "usb";
+		status = "okay";
+
+		resets = <&powerdown STIH416_USB0_POWERDOWN>,
+			 <&softreset STIH416_USB0_SOFTRESET>;
+		reset-names = "power", "softreset";
+	};