Documentation: gpio: Explain that <function>-gpio is also supported

The GPIO documentation mentions that GPIOs are mapped by defining a
<function>-gpios property in the consumer device's node but a -gpio
sufix is also supported after commit:

dd34c37aa3e8 ("gpio: of: Allow -gpio suffix for property names")

Update the documentation to match the implementation.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/Documentation/gpio/board.txt b/Documentation/gpio/board.txt
index 5fa069a..f59c43b 100644
--- a/Documentation/gpio/board.txt
+++ b/Documentation/gpio/board.txt
@@ -21,8 +21,8 @@
 device tree bindings for your controller.
 
 GPIOs mappings are defined in the consumer device's node, in a property named
-<function>-gpios, where <function> is the function the driver will request
-through gpiod_get(). For example:
+either <function>-gpios or <function>-gpio, where <function> is the function
+the driver will request through gpiod_get(). For example:
 
 	foo_device {
 		compatible = "acme,foo";
@@ -31,7 +31,7 @@
 			    <&gpio 16 GPIO_ACTIVE_HIGH>, /* green */
 			    <&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */
 
-		power-gpios = <&gpio 1 GPIO_ACTIVE_LOW>;
+		power-gpio = <&gpio 1 GPIO_ACTIVE_LOW>;
 	};
 
 This property will make GPIOs 15, 16 and 17 available to the driver under the