of-bindings: Add binding documentation for SPI busses and devices

Add documentation about how to describe SPI busses in the device tree.
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>

Documentation/powerpc/booting-without-of.txt

@@ -59,6 +59,7 @@ Table of Contents
       p) Freescale Synchronous Serial Interface
 	  q) USB EHCI controllers
       r) MDIO on GPIOs
+      s) SPI busses
 
   VII - Marvell Discovery mv64[345]6x System Controller chips
     1) The /system-controller node
@@ -1881,6 +1882,62 @@ platforms are moved over to use the flattened-device-tree model.
 			 &qe_pio_c 6>;
 	};
 
+    s) SPI (Serial Peripheral Interface) busses
+
+    SPI busses can be described with a node for the SPI master device
+    and a set of child nodes for each SPI slave on the bus.  For this
+    discussion, it is assumed that the system's SPI controller is in
+    SPI master mode.  This binding does not describe SPI controllers
+    in slave mode.
+
+    The SPI master node requires the following properties:
+    - #address-cells  - number of cells required to define a chip select
+			address on the SPI bus.
+    - #size-cells     - should be zero.
+    - compatible      - name of SPI bus controller following generic names
+			recommended practice.
+    No other properties are required in the SPI bus node.  It is assumed
+    that a driver for an SPI bus device will understand that it is an SPI bus.
+    However, the binding does not attempt to define the specific method for
+    assigning chip select numbers.  Since SPI chip select configuration is
+    flexible and non-standardized, it is left out of this binding with the
+    assumption that board specific platform code will be used to manage
+    chip selects.  Individual drivers can define additional properties to
+    support describing the chip select layout.
+
+    SPI slave nodes must be children of the SPI master node and can
+    contain the following properties.
+    - reg             - (required) chip select address of device.
+    - compatible      - (required) name of SPI device following generic names
+			recommended practice
+    - spi-max-frequency - (required) Maximum SPI clocking speed of device in Hz
+    - spi-cpol        - (optional) Empty property indicating device requires
+			inverse clock polarity (CPOL) mode
+    - spi-cpha        - (optional) Empty property indicating device requires
+			shifted clock phase (CPHA) mode
+
+    SPI example for an MPC5200 SPI bus:
+		spi@f00 {
+			#address-cells = <1>;
+			#size-cells = <0>;
+			compatible = "fsl,mpc5200b-spi","fsl,mpc5200-spi";
+			reg = <0xf00 0x20>;
+			interrupts = <2 13 0 2 14 0>;
+			interrupt-parent = <&mpc5200_pic>;
+
+			ethernet-switch@0 {
+				compatible = "micrel,ks8995m";
+				spi-max-frequency = <1000000>;
+				reg = <0>;
+			};
+
+			codec@1 {
+				compatible = "ti,tlv320aic26";
+				spi-max-frequency = <100000>;
+				reg = <1>;
+			};
+		};
+
 VII - Marvell Discovery mv64[345]6x System Controller chips
 ===========================================================