staging: mt7621-gpio: dt-bindings: complete documentation for the gpio

  - "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

- interrupts : Interrupt specifier for the controllers interrupt.

- interrupt-controller : Mark the device node as an interrupt controller.

- #interrupt-cells : Should be 1. The first cell is the GPIO number.

Required properties for the GPIO bank node:

- compatible:

  - "mediatek,mt7621-gpio-bank" for Mediatek banks

- #gpio-cells : Should be two.

  - first cell is the pin number

  - second cell is used to specify optional parameters (unused)

- gpio-controller : Marks the device node as a GPIO controller

- #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.

- gpio-controller : Marks the device node as a GPIO controller.

- reg : The id of the bank that the node describes.

Example:

		interrupt-parent = <&gic>;

		interrupts = <GIC_SHARED 12 IRQ_TYPE_LEVEL_HIGH>;

		interrupt-controller;

		#interrupt-cells = <1>;

		gpio0: bank@0 {

			reg = <0>;