iio:as3935: Add DT binding docs for AS3935 driver

Document compatible string, required and optional DT properties for
AS3935 chipset driver.
Signed-off-by: NMatt Ranostay <mranostay@gmail.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/proximity/as3935.txt

+Austrian Microsystems AS3935 Franklin lightning sensor device driver
+
+Required properties:
+	- compatible: must be "ams,as3935"
+	- reg: SPI chip select number for the device
+	- spi-cpha: SPI Mode 1. Refer to spi/spi-bus.txt for generic SPI
+	slave node bindings.
+	- interrupt-parent : should be the phandle for the interrupt controller
+	- interrupts : the sole interrupt generated by the device
+
+	Refer to interrupt-controller/interrupts.txt for generic
+	interrupt client node bindings.
+
+Optional properties:
+	- ams,tuning-capacitor-pf: Calibration tuning capacitor stepping
+	  value 0 - 120pF. This will require using the calibration data from
+	  the manufacturer.
+
+Example:
+
+as3935@0 {
+	compatible = "ams,as3935";
+	reg = <0>;
+	spi-cpha;
+	interrupt-parent = <&gpio1>;
+	interrupts = <16 1>;
+	ams,tuning-capacitor-pf = <80>;
+};

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -11,6 +11,7 @@ ak	Asahi Kasei Corp.
 allwinner	Allwinner Technology Co., Ltd.
 altr	Altera Corp.
 amcc	Applied Micro Circuits Corporation (APM, formally AMCC)
+ams	AMS AG
 amstaos	AMS-Taos Inc.
 apm	Applied Micro Circuits Corporation (APM)
 arm	ARM Ltd.