ARM: mvebu: Add a 2 GHz fixed-clock Armada 370/XP

Armada 370/XP SoCs have a 2 GHz fixed PLL that is used to feed
other clocks. This commit adds a DT representation of this clock
through a fixed-clock compatible node.
Reviewed-by: NGregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: NEzequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: NJason Cooper <jason@lakedaemon.net>

arch/arm/boot/dts/armada-370-xp.dtsi

@@ -251,4 +251,13 @@
 
 		};
 	};
+
+	clocks {
+		/* 2 GHz fixed main PLL */
+		mainpll: mainpll {
+			compatible = "fixed-clock";
+			#clock-cells = <0>;
+			clock-frequency = <2000000000>;
+		};
+	};
  };