Documentation: bindings: add the marvell,berlin-smp CPU enable method

Document the CPU enable method used by Marvell Berlin SoCs.
Signed-off-by: Antoine Ténart <antoine.tenart@free-electrons.com>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>

Documentation/devicetree/bindings/arm/cpu-enable-method/marvell,berlin-smp

+========================================================
+Secondary CPU enable-method "marvell,berlin-smp" binding
+========================================================
+
+This document describes the "marvell,berlin-smp" method for enabling secondary
+CPUs. To apply to all CPUs, a single "marvell,berlin-smp" enable method should
+be defined in the "cpus" node.
+
+Enable method name:	"marvell,berlin-smp"
+Compatible machines:	"marvell,berlin2" and "marvell,berlin2q"
+Compatible CPUs:	"marvell,pj4b" and "arm,cortex-a9"
+Related properties:	(none)
+
+Note:
+This enable method needs valid nodes compatible with "arm,cortex-a9-scu" and
+"marvell,berlin-cpu-ctrl"[1].
+
+Example:
+
+	cpus {
+		#address-cells = <1>;
+		#size-cells = <0>;
+		enable-method = "marvell,berlin-smp";
+
+		cpu@0 {
+			compatible = "marvell,pj4b";
+			device_type = "cpu";
+			next-level-cache = <&l2>;
+			reg = <0>;
+		};
+
+		cpu@1 {
+			compatible = "marvell,pj4b";
+			device_type = "cpu";
+			next-level-cache = <&l2>;
+			reg = <1>;
+		};
+	};
+
+--
+[1] arm/marvell,berlin.txt