dt-bindings: input: Add DT bindings for max77650

Add the DT binding document for the onkey module of max77650.
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/input/max77650-onkey.txt

+Onkey driver for MAX77650 PMIC from Maxim Integrated.
+
+This module is part of the MAX77650 MFD device. For more details
+see Documentation/devicetree/bindings/mfd/max77650.txt.
+
+The onkey controller is represented as a sub-node of the PMIC node on
+the device tree.
+
+Required properties:
+--------------------
+- compatible:		Must be "maxim,max77650-onkey".
+
+Optional properties:
+- linux,code:		The key-code to be reported when the key is pressed.
+			Defaults to KEY_POWER.
+- maxim,onkey-slide:	The system's button is a slide switch, not the default
+			push button.
+
+Example:
+--------
+
+	onkey {
+		compatible = "maxim,max77650-onkey";
+		linux,code = <KEY_END>;
+		maxim,onkey-slide;
+	};