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

Add Device Tree binding document for GE Healthcare USB Management
Controller (ACHC).
Signed-off-by: NMartyn Welch <martyn.welch@collabora.co.uk>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/misc/ge-achc.txt

+* 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>;
+};