Mediatek MT7621 SoC GPIO controller bindings

The IP core used inside these SoCs has 3 banks of 32 GPIOs each.
The registers of all the banks are interwoven inside one single IO range.
We load one GPIO controller instance per bank. Also the GPIO controller can receive
interrupts on any of the GPIOs, either edge or level. It then interrupts the CPU
using GIC INT12.

Required properties for the top level node:
- #gpio-cells : Should be two. The first cell is the GPIO pin number and the
   second cell specifies GPIO flags, as defined in <dt-bindings/gpio/gpio.h>.
   Only the GPIO_ACTIVE_HIGH and GPIO_ACTIVE_LOW flags are supported.
- #interrupt-cells : Specifies the number of cells needed to encode an
   interrupt. Should be 2. The first cell defines the interrupt number,
   the second encodes the triger flags encoded as described in
   Documentation/devicetree/bindings/interrupt-controller/interrupts.txt
- compatible:
  - "mediatek,mt7621-gpio" for Mediatek controllers
- reg : Physical base address and length of the controller's registers
- interrupt-parent : phandle of the parent interrupt controller.
- interrupts : Interrupt specifier for the controllers interrupt.
- interrupt-controller : Mark the device node as an interrupt controller.
- gpio-controller : Marks the device node as a GPIO controller.

Example:
	gpio@600 {
		#gpio-cells = <2>;
		#interrupt-cells = <2>;
		compatible = "mediatek,mt7621-gpio";
		gpio-controller;
		interrupt-controller;
		reg = <0x600 0x100>;
		interrupt-parent = <&gic>;
		interrupts = <GIC_SHARED 12 IRQ_TYPE_LEVEL_HIGH>;
	};