mfd: lochnagar: Add initial binding documentation

Lochnagar is an evaluation and development board for Cirrus
Logic Smart CODEC and Amp devices. It allows the connection of
most Cirrus Logic devices on mini-cards, as well as allowing
connection of various application processor systems to provide a
full evaluation platform. This driver supports the board
controller chip on the Lochnagar board.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/cirrus,lochnagar.txt

@@ -0,0 +1,68 @@
+Cirrus Logic Lochnagar Audio Development Board
+
+Lochnagar is an evaluation and development board for Cirrus Logic
+Smart CODEC and Amp devices. It allows the connection of most Cirrus
+Logic devices on mini-cards, as well as allowing connection of
+various application processor systems to provide a full evaluation
+platform.  Audio system topology, clocking and power can all be
+controlled through the Lochnagar, allowing the device under test
+to be used in a variety of possible use cases.
+
+Also see these documents for generic binding information:
+  [1] GPIO : ../gpio/gpio.txt
+
+And these for relevant defines:
+  [2] include/dt-bindings/pinctrl/lochnagar.h
+  [3] include/dt-bindings/clock/lochnagar.h
+
+And these documents for the required sub-node binding details:
+  [4] Clock: ../clock/cirrus,lochnagar.txt
+  [5] Pinctrl: ../pinctrl/cirrus,lochnagar.txt
+  [6] Regulator: ../regulator/cirrus,lochnagar.txt
+
+Required properties:
+
+  - compatible : One of the following strings:
+                 "cirrus,lochnagar1"
+                 "cirrus,lochnagar2"
+
+  - reg : I2C slave address
+
+  - reset-gpios : Reset line to the Lochnagar, see [1].
+
+Required sub-nodes:
+
+  - lochnagar-clk : Binding for the clocking components, see [4].
+
+  - lochnagar-pinctrl : Binding for the pin control components, see [5].
+
+Optional sub-nodes:
+
+  - Bindings for the regulator components, see [6]. Only available on
+    Lochnagar 2.
+
+Optional properties:
+
+  - present-gpios : Host present line, indicating the presence of a
+    host system, see [1]. This can be omitted if the present line is
+    tied in hardware.
+
+Example:
+
+lochnagar: lochnagar@22 {
+	compatible = "cirrus,lochnagar2";
+	reg = <0x22>;
+
+	reset-gpios = <&gpio0 55 0>;
+	present-gpios = <&gpio0 60 0>;
+
+	lochnagar-clk {
+		compatible = "cirrus,lochnagar2-clk";
+		...
+	};
+
+	lochnagar-pinctrl {
+		compatible = "cirrus,lochnagar-pinctrl";
+		...
+	};
+};