dt-bindings: add Marvell PMU documentation

Add the required DT binding documentation for the Marvell PMU driver.
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>

Documentation/devicetree/bindings/soc/dove/pmu.txt

+Device Tree bindings for Marvell PMU
+
+Required properties:
+ - compatible: value should be "marvell,dove-pmu".
+    May also include "simple-bus" if there are child devices, in which
+    case the ranges node is required.
+ - reg: two base addresses and sizes of the PM controller and PMU.
+ - interrupts: single interrupt number for the PMU interrupt
+ - interrupt-controller: must be specified as the PMU itself is an
+    interrupt controller.
+ - #interrupt-cells: must be 1.
+ - #reset-cells: must be 1.
+ - domains: sub-node containing domain descriptions
+
+Optional properties:
+ - ranges: defines the address mapping for child devices, as per the
+   standard property of this name.  Required when compatible includes
+   "simple-bus".
+
+Power domain descriptions are listed as child nodes of the "domains"
+sub-node.  Each domain has the following properties:
+
+Required properties:
+ - #power-domain-cells: must be 0.
+
+Optional properties:
+ - marvell,pmu_pwr_mask: specifies the mask value for PMU power register
+ - marvell,pmu_iso_mask: specifies the mask value for PMU isolation register
+ - resets: points to the reset manager (PMU node) and reset index.
+
+Example:
+
+	pmu: power-management@d0000 {
+		compatible = "marvell,dove-pmu";
+		reg = <0xd0000 0x8000>, <0xd8000 0x8000>;
+		interrupts = <33>;
+		interrupt-controller;
+		#interrupt-cells = <1>;
+		#reset-cells = <1>;
+
+		domains {
+			vpu_domain: vpu-domain {
+				#power-domain-cells = <0>;
+				marvell,pmu_pwr_mask = <0x00000008>;
+				marvell,pmu_iso_mask = <0x00000001>;
+				resets = <&pmu 16>;
+			};
+
+			gpu_domain: gpu-domain {
+				#power-domain-cells = <0>;
+				marvell,pmu_pwr_mask = <0x00000004>;
+				marvell,pmu_iso_mask = <0x00000002>;
+				resets = <&pmu 18>;
+			};
+		};
+	};