dt-bindings: bus: Add documentation for the Technologic Systems NBUS

Add binding documentation for the Technologic Systems NBUS that is used
to interface with peripherals in the FPGA of the TS-4600 SoM.
Signed-off-by: Sebastien Bourdelin <sebastien.bourdelin@savoirfairelinux.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Documentation/devicetree/bindings/bus/ts-nbus.txt

+Technologic Systems NBUS
+
+The NBUS is a bus used to interface with peripherals in the Technologic
+Systems FPGA on the TS-4600 SoM.
+
+Required properties :
+ - compatible		: "technologic,ts-nbus"
+ - #address-cells	: must be 1
+ - #size-cells		: must be 0
+ - pwms			: The PWM bound to the FPGA
+ - ts,data-gpios	: The 8 GPIO pins connected to the data lines on the FPGA
+ - ts,csn-gpios		: The GPIO pin connected to the csn line on the FPGA
+ - ts,txrx-gpios	: The GPIO pin connected to the txrx line on the FPGA
+ - ts,strobe-gpios	: The GPIO pin connected to the stobe line on the FPGA
+ - ts,ale-gpios		: The GPIO pin connected to the ale line on the FPGA
+ - ts,rdy-gpios		: The GPIO pin connected to the rdy line on the FPGA
+
+Child nodes:
+
+The NBUS node can contain zero or more child nodes representing peripherals
+on the bus.
+
+Example:
+
+	nbus {
+		compatible = "technologic,ts-nbus";
+		pinctrl-0 = <&nbus_pins>;
+		#address-cells = <1>;
+		#size-cells = <0>;
+		pwms = <&pwm 2 83>;
+		ts,data-gpios   = <&gpio0 0 GPIO_ACTIVE_HIGH
+				   &gpio0 1 GPIO_ACTIVE_HIGH
+				   &gpio0 2 GPIO_ACTIVE_HIGH
+				   &gpio0 3 GPIO_ACTIVE_HIGH
+				   &gpio0 4 GPIO_ACTIVE_HIGH
+				   &gpio0 5 GPIO_ACTIVE_HIGH
+				   &gpio0 6 GPIO_ACTIVE_HIGH
+				   &gpio0 7 GPIO_ACTIVE_HIGH>;
+		ts,csn-gpios    = <&gpio0 16 GPIO_ACTIVE_HIGH>;
+		ts,txrx-gpios   = <&gpio0 24 GPIO_ACTIVE_HIGH>;
+		ts,strobe-gpios = <&gpio0 25 GPIO_ACTIVE_HIGH>;
+		ts,ale-gpios    = <&gpio0 26 GPIO_ACTIVE_HIGH>;
+		ts,rdy-gpios    = <&gpio0 21 GPIO_ACTIVE_HIGH>;
+
+		watchdog@2a {
+			compatible = "...";
+
+			/* ... */
+		};
+	};