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

Optional properties:
- assert-falling-edge: when present, assert is indicated by a falling edge
                       (instead of by a rising edge)
- use-system-time-ts: use the system time via pps_get_ts as the timestamp
                      if this is not defined then the timestamp will come
                      from the monotonic boot time

Example:
	pps {
		compatible = "pps-gpio";
		gpios = <&gpio2 6 0>;

		assert-falling-edge;
	};