gpio: add DT binding doc for gpio of PMIC max77620/max20024

GPIO driver for MAX77620 Power management IC from Maxim Semiconductor.

Device has 8 GPIO pins which can be configured as GPIO as well as the

special IO functions.

Required properties:

-------------------

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

- #gpio-cells : 	Should be two.  The first cell is the pin number and

			the second cell is used to specify the gpio polarity:

				0 = active high

				1 = active low

For more details, please refer generic GPIO DT binding document

<devicetree/bindings/gpio/gpio.txt>.

Example:

--------

#include <dt-bindings/mfd/max77620.h>

...

max77620@3c {

	compatible = "maxim,max77620";

	gpio-controller;

	#gpio-cells = <2>;

};