dt-bindings: bus: Document moxtet bus binding

Turris Mox module status and configuration bus (over SPI)

Required properties:

 - compatible		: Should be "cznic,moxtet"

 - #address-cells	: Has to be 1

 - #size-cells		: Has to be 0

 - spi-cpol		: Required inverted clock polarity

 - spi-cpha		: Required shifted clock phase

 - interrupts		: Must contain reference to the shared interrupt line

 - interrupt-controller	: Required

 - #interrupt-cells	: Has to be 1

For other required and optional properties of SPI slave nodes please refer to

../spi/spi-bus.txt.

Required properties of subnodes:

 - reg			: Should be position on the Moxtet bus (how many Moxtet

			  modules are between this module and CPU module, so

			  either 0 or a positive integer)

The driver finds the devices connected to the bus by itself, but it may be

needed to reference some of them from other parts of the device tree. In that

case the devices can be defined as subnodes of the moxtet node.

Example:

	moxtet@1 {

		compatible = "cznic,moxtet";

		#address-cells = <1>;

		#size-cells = <0>;

		reg = <1>;

		spi-max-frequency = <10000000>;

		spi-cpol;

		spi-cpha;

		interrupt-controller;

		#interrupt-cells = <1>;

		interrupt-parent = <&gpiosb>;

		interrupts = <5 IRQ_TYPE_EDGE_FALLING>;

		moxtet_sfp: gpio@0 {

			compatible = "cznic,moxtet-gpio";

			gpio-controller;

			#gpio-cells = <2>;

			reg = <0>;

		}

	};

M:	Marek Behun <marek.behun@nic.cz>

W:	http://mox.turris.cz

S:	Maintained

F:	Documentation/devicetree/bindings/bus/moxtet.txt

F:	include/linux/moxtet.h

F:	drivers/bus/moxtet.c