dt-bindings: arm: add DT binding for Marvell AP806 system controller

This commit adds the Device Tree binding documentation for the system
controller found in Marvell AP806 HW block, which is one of the core
HW blocks of the 64-bits Marvell Armada 7K/8K family.
Signed-off-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NStephen Boyd <sboyd@codeaurora.org>

Documentation/devicetree/bindings/arm/marvell/ap806-system-controller.txt

+Marvell Armada AP806 System Controller
+======================================
+
+The AP806 is one of the two core HW blocks of the Marvell Armada 7K/8K
+SoCs. It contains a system controller, which provides a number
+registers giving access to numerous features: clocks, pin-muxing and
+many other SoC configuration items. This DT binding allows to describe
+this system controller.
+
+The Device Tree node representing the AP806 system controller provides
+a number of clocks:
+
+ - 0: clock of CPU cluster 0
+ - 1: clock of CPU cluster 1
+ - 2: fixed PLL at 1200 Mhz
+ - 3: MSS clock, derived from the fixed PLL
+
+Required properties:
+
+ - compatible: must be:
+     "marvell,ap806-system-controller", "syscon"
+ - reg: register area of the AP806 system controller
+ - #clock-cells: must be set to 1
+ - clock-output-names: must be defined to:
+    "ap-cpu-cluster-0", "ap-cpu-cluster-1", "ap-fixed", "ap-mss"
+
+Example:
+
+	syscon: system-controller@6f4000 {
+		compatible = "marvell,ap806-system-controller", "syscon";
+		#clock-cells = <1>;
+		clock-output-names = "ap-cpu-cluster-0", "ap-cpu-cluster-1",
+				     "ap-fixed", "ap-mss";
+		reg = <0x6f4000 0x1000>;
+	};