dt-bindings: mfd: da9062: add gpio bindings

Add gpio device documentation to make the da9062 gpios available for
users.
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/da9062.txt

@@ -13,6 +13,7 @@ da9062-rtc              :               : Real-Time Clock
 da9062-onkey            :               : On Key
 da9062-watchdog         :               : Watchdog Timer
 da9062-thermal          :               : Thermal
+da9062-gpio             :               : GPIOs
 
 The DA9061 PMIC consists of:
 
@@ -38,6 +39,15 @@ Required properties:
 See Documentation/devicetree/bindings/interrupt-controller/interrupts.txt for
 further information on IRQ bindings.
 
+Optional properties:
+
+- gpio-controller : Marks the device 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.
+
+See Documentation/devicetree/bindings/gpio/gpio.txt for further information on
+GPIO bindings.
+
 Sub-nodes:
 
 - regulators : This node defines the settings for the LDOs and BUCKs.