staging: mt7621-gpio: dt-bindings: update documentation for #interrupt-cells property

- 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.

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

- #interrupt-cells : Should be 2. The first cell defines the interrupt number.

   The second cell bits[3:0] is used to specify trigger type as follows:

	- 1 = low-to-high edge triggered.

	- 2 = high-to-low edge triggered.

	- 4 = active high level-sensitive.

	- 8 = active low level-sensitive.

Required properties for the GPIO bank node:

- compatible:

		interrupt-parent = <&gic>;

		interrupts = <GIC_SHARED 12 IRQ_TYPE_LEVEL_HIGH>;

		interrupt-controller;

		#interrupt-cells = <1>;

		#interrupt-cells = <2>;

		gpio0: bank@0 {

			reg = <0>;