dt-bindings: pps: pps-gpio PPS ECHO implementation

This patch implements the device tree binding changes required for the
PPS ECHO functionality for pps-gpio, that sysfs claims is available
already.

It adds two DT properties for configuring the PPS ECHO functionality.

This patch is provided separated from the rest of the patch per
Documentation/devicetree/bindings/submitting-patches.txt.

This patch was originally written by Lukas Senger as part of a masters
thesis project and modified for inclusion into the linux kernel by Tom
Burkart.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Tom Burkart <[e-mail]>
Signed-off-by: Lukas Senger <[e-mail]>
Acked-by: Rodolfo Giometti <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Cc: Philipp Zabel <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/devicetree/bindings/pps/pps-gpio.txt

···
 - 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)
···
 
 		gpios = <&gpio1 26 GPIO_ACTIVE_HIGH>;
 		assert-falling-edge;
+
+		echo-gpios = <&gpio1 27 GPIO_ACTIVE_HIGH>;
+		echo-active-ms = <100>;
 
 		compatible = "pps-gpio";
 	};