dt-bindings: misc: bk4: Add device tree binding for Liebherr's BK4 SPI bus

Add Device Tree binding document for Liebherr's BK4 external SPI bus.
Signed-off-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/misc/lwn-bk4.txt

+* Liebherr's BK4 controller external SPI
+
+A device which handles data acquisition from compatible industrial
+peripherals.
+The SPI is used for data and management purposes in both master and
+slave modes.
+
+Required properties:
+
+- compatible : Should be "lwn,bk4"
+
+Required SPI properties:
+
+- reg : Should be address of the device chip select within
+  the controller.
+
+- spi-max-frequency : Maximum SPI clocking speed of device in Hz, should be
+  30MHz at most for the Liebherr's BK4 external bus.
+
+Example:
+
+spidev0: spi@0 {
+	compatible = "lwn,bk4";
+	spi-max-frequency = <30000000>;
+	reg = <0>;
+};