devicetree: bindings: add missing Marvell Dove SoC documentation

Marvell Dove SoC binding was not documented, yet. Add the documentation
and also describe Global Configuration register node in it.
Signed-off-by: NSebastian Hesselbarth <sebastian.hesselbarth@gmail.com>

Documentation/devicetree/bindings/arm/marvell,dove.txt

+Marvell Dove Platforms Device Tree Bindings
+-----------------------------------------------
+
+Boards with a Marvell Dove SoC shall have the following properties:
+
+Required root node property:
+- compatible: must contain "marvell,dove";
+
+* Global Configuration registers
+
+Global Configuration registers of Dove SoC are shared by a syscon node.
+
+Required properties:
+- compatible: must contain "marvell,dove-global-config" and "syscon".
+- reg: base address and size of the Global Configuration registers.
+
+Example:
+
+gconf: global-config@e802c {
+	compatible = "marvell,dove-global-config", "syscon";
+	reg = <0xe802c 0x14>;
+};