ARM: mvebu: Add the reference 25 MHz fixed-clock to Armada XP

The Armada XP SoC has a reference 25 MHz fixed-clock that is used in
some controllers such as the timer and the watchdog. This commit adds
a DT representation of this clock through a fixed-clock compatible node.
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Reviewed-by: Mike Turquette <mturquette@linaro.org>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>

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

@@ -169,4 +169,13 @@ thermal@182b0 {
 			};
 		};
 	};
+
+	clocks {
+		/* 25 MHz reference crystal */
+		refclk: oscillator {
+			compatible = "fixed-clock";
+			#clock-cells = <0>;
+			clock-frequency = <25000000>;
+		};
+	};
 };