Device-Tree Bindings for a PPS Signal on GPIO

These properties describe a PPS (pulse-per-second) signal connected to
a GPIO pin.

Required properties:
- compatible: should be "pps-gpio"
- gpios: one PPS GPIO in the format described by ../gpio/gpio.txt

Additional required properties for the PPS ECHO functionality:
- echo-gpios: one PPS ECHO GPIO in the format described by ../gpio/gpio.txt
- echo-active-ms: duration in ms of the active portion of the echo pulse

Optional properties:
- assert-falling-edge: when present, assert is indicated by a falling edge
                       (instead of by a rising edge)

Example:
	pps {
		pinctrl-names = "default";
		pinctrl-0 = <&pinctrl_pps>;

		gpios = <&gpio1 26 GPIO_ACTIVE_HIGH>;
		assert-falling-edge;

		echo-gpios = <&gpio1 27 GPIO_ACTIVE_HIGH>;
		echo-active-ms = <100>;

		compatible = "pps-gpio";
	};