dt-bindings: input: Document STPMIC1 PMIC onkey

The STPMIC1 PMIC is able to manage an onkey button. It can be configured
to shut-down the power supplies on a long key-press with an adjustable
duration.
Signed-off-by: Pascal Paillet <p.paillet@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/input/st,stpmic1-onkey.txt

+STMicroelectronics STPMIC1 Onkey
+
+Required properties:
+
+- compatible = "st,stpmic1-onkey";
+- interrupts: interrupt line to use
+- interrupt-names = "onkey-falling", "onkey-rising"
+	onkey-falling: happens when onkey is pressed; IT_PONKEY_F of pmic
+	onkey-rising: happens when onkey is released; IT_PONKEY_R of pmic
+
+Optional properties:
+
+- st,onkey-clear-cc-flag: onkey is able power on after an
+  over-current shutdown event.
+- st,onkey-pu-inactive: onkey pull up is not active
+- power-off-time-sec: Duration in seconds which the key should be kept
+        pressed for device to power off automatically (from 1 to 16 seconds).
+        see See Documentation/devicetree/bindings/input/keys.txt
+
+Example:
+
+onkey {
+	compatible = "st,stpmic1-onkey";
+	interrupt-parent = <&pmic>;
+	interrupts = <IT_PONKEY_F 0>,<IT_PONKEY_R 1>;
+	interrupt-names = "onkey-falling", "onkey-rising";
+	power-off-time-sec = <10>;
+};