ARM: dts: dm8168-evm: add the external reference clock for SATA

This board has an external oscillator supplying the reference clock
signal for SATA. Its rate is fixed at 100Mhz. Add a corresponding
device tree node.
Signed-off-by: NBartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: NTony Lindgren <tony@atomide.com>

arch/arm/boot/dts/dm8168-evm.dts

@@ -25,6 +25,12 @@
 		regulator-min-microvolt = <3300000>;
 		regulator-max-microvolt = <3300000>;
 	};
+
+	sata_refclk: fixedclock0 {
+		compatible = "fixed-clock";
+		#clock-cells = <0>;
+		clock-frequency = <100000000>;
+	};
 };
 
 &dm816x_pinmux {