Cavium ThunderX/OCTEON-TX GPIO controller bindings

Required Properties:
- reg: The controller bus address.
- gpio-controller: Marks the device node as a GPIO controller.
- #gpio-cells: Must be 2.
  - First cell is the GPIO pin number relative to the controller.
  - Second cell is a standard generic flag bitfield as described in gpio.txt.

Optional Properties:
- compatible: "cavium,thunder-8890-gpio", unused as PCI driver binding is used.
- interrupt-controller: Marks the device node as an interrupt controller.
- #interrupt-cells: Must be present and have value of 2 if
                    "interrupt-controller" is present.
  - First cell is the GPIO pin number relative to the controller.
  - Second cell is triggering flags as defined in interrupts.txt.

Example:

gpio_6_0: gpio@6,0 {
	compatible = "cavium,thunder-8890-gpio";
	reg = <0x3000 0 0 0 0>; /*  DEVFN = 0x30 (6:0) */
	gpio-controller;
	#gpio-cells = <2>;
	interrupt-controller;
	#interrupt-cells = <2>;
};