gpio: Document GPIO hogging mechanism

property, and a #gpio-cells integer property, which indicates the number of

cells in a gpio-specifier.

The GPIO chip may contain GPIO hog definitions. GPIO hogging is a mechanism

providing automatic GPIO request and configuration as part of the

gpio-controller's driver probe function.

Each GPIO hog definition is represented as a child node of the GPIO controller.

Required properties:

- gpio-hog:   A property specifying that this child node represent a GPIO hog.

- gpios:      Store the GPIO information (id, flags, ...). Shall contain the

	      number of cells specified in its parent node (GPIO controller

	      node).

Only one of the following properties scanned in the order shown below.

This means that when multiple properties are present they will be searched

in the order presented below and the first match is taken as the intended

configuration.

- input:      A property specifying to set the GPIO direction as input.

- output-low  A property specifying to set the GPIO direction as output with

	      the value low.

- output-high A property specifying to set the GPIO direction as output with

	      the value high.

Optional properties:

- line-name:  The GPIO label name. If not present the node name is used.

Example of two SOC GPIO banks defined as gpio-controller nodes:

	qe_pio_a: gpio-controller@1400 {

		reg = <0x1400 0x18>;

		gpio-controller;

		#gpio-cells = <2>;

		line_b {

			gpio-hog;

			gpios = <6 0>;

			output-low;

			line-name = "foo-bar-gpio";

		};

	};

	qe_pio_e: gpio-controller@1460 {