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

* 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>;

};