ARM: at91/dt: define main xtal frequency of the at91sam9261ek board

Define at91sam9261ek main crystal frequency.
Signed-off-by: NBoris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: NJean-Jacques HIBLOT <jjhiblot@traphandler.com>
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/at91sam9261ek.dts

@@ -20,6 +20,10 @@
 		reg = <0x20000000 0x4000000>;
 	};
 
+	main_xtal {
+		clock-frequency = <18432000>;
+	};
+
 	clocks {
 		#address-cells = <1>;
 		#size-cells = <1>;