dt-bindings: misc: achc: Add device tree binding for GE ACHC

* GE Healthcare USB Management Controller

A device which handles data aquisition from compatible USB based peripherals.

SPI is used for device management.

Note: This device does not expose the peripherals as USB devices.

Required properties:

- compatible : Should be "ge,achc"

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

  1MHz for the GE ACHC.

Example:

spidev0: spi@0 {

	compatible = "ge,achc";

	reg = <0>;

	spi-max-frequency = <1000000>;

};