dt-bindings: max3421: Add bindings documentation

Adds bindings documentation for the max3421 driver.
Signed-off-by: Jules Maselbas <jules.maselbas@grenoble-inp.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/maxim,max3421.txt

+Maxim Integrated SPI-based USB 2.0 host controller MAX3421E
+
+Required properties:
+ - compatible: "maxim,max3421"
+ - spi-max-frequency: maximum frequency for this device must not exceed 26 MHz.
+ - reg: chip select number to which this device is connected.
+ - maxim,vbus-en-pin: <GPOUTx ACTIVE_LEVEL>
+   GPOUTx is the number (1-8) of the GPOUT pin of MAX3421E to drive Vbus.
+   ACTIVE_LEVEL is 0 or 1.
+ - interrupt-parent: the phandle of the associated interrupt controller.
+ - interrupts: the interuption line description for the interrupt controller.
+   The driver configures MAX3421E for active low level triggered interrupts,
+   configure your interrupt line accordingly.
+
+Example:
+
+	usb@0 {
+		compatible = "maxim,max3421";
+		reg = <0>;
+		maxim,vbus-en-pin = <3 1>;
+		spi-max-frequency = <26000000>;
+		interrupt-parent = <&PIC>;
+		interrupts = <42>;
+	};