Austrian Microsystems AS3935 Franklin lightning sensor device driver

Required properties:
	- compatible: must be "ams,as3935"
	- reg: SPI chip select number for the device
	- spi-max-frequency: specifies maximum SPI clock frequency
	- spi-cpha: SPI Mode 1. Refer to spi/spi-bus.txt for generic SPI
	slave node bindings.
	- 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.
	- ams,nflwdth: Set the noise and watchdog threshold register on
	  startup. This will need to set according to the noise from the
	  MCU board, and possibly the local environment. Refer to the
	  datasheet for the threshold settings.

Example:

as3935@0 {
	compatible = "ams,as3935";
	reg = <0>;
	spi-max-frequency = <400000>;
	spi-cpha;
	interrupt-parent = <&gpio1>;
	interrupts = <16 1>;
	ams,tuning-capacitor-pf = <80>;
	ams,nflwdth = <0x44>;
};