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

Maxim Semiconductor's PMIC MAX77620/MAX20024 has 8 GPIO pins
which act as GPIO as well as special function mode.

Add DT binding document to support these pins in GPIO
mode via GPIO framework.
Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/gpio-max77620.txt

+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>;
+};