SiFive GPIO controller bindings

Required properties:
- compatible:
  - "sifive,gpio0"
- reg: Physical base address and length of the controller's registers.
- #gpio-cells : Should be 2
  - The first cell is the gpio offset number.
  - The second cell indicates the polarity of the GPIO
- gpio-controller : Marks the device node as a GPIO controller.
- interrupt-controller: Mark the device node as an interrupt controller
- #interrupt-cells : Should be 2.
  - The first cell is the GPIO offset number within the GPIO controller.
  - The second cell is the edge/level to use for interrupt generation.
- interrupts: Specify the interrupts, one per GPIO

Example:

gpio: gpio@10060000 {
	compatible = "sifive,gpio0";
	interrupt-parent = <&plic>;
	interrupts = <7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22>;
	reg = <0x0 0x10060000 0x0 0x1000>;
	gpio-controller;
	#gpio-cells = <2>;
	interrupt-controller;
	#interrupt-cells = <2>;
};
