dt-bindings: bus: Document moxtet bus binding

This adds device tree binding documentation for the Moxtet bus, a bus
via which the different modules connected to the Turris Mox router can
be configured.

Link: https://lore.kernel.org/r/20190812161118.21476-3-marek.behun@nic.czSigned-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/bus/moxtet.txt

+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>;
+		}
+	};

MAINTAINERS

@@ -1630,6 +1630,7 @@ ARM/CZ.NIC TURRIS MOX SUPPORT
 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