dt-bindings: input: Document STPMIC1 PMIC onkey

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>;

};