ARM: at91/dt: define sam9n12ek crystal frequencies

Define sam9n12ek's main and slow crystal frequencies.
Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Tested-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/at91sam9n12ek.dts

@@ -21,6 +21,14 @@ memory {
 		reg = <0x20000000 0x8000000>;
 	};
 
+	slow_xtal {
+		clock-frequency = <32768>;
+	};
+
+	main_xtal {
+		clock-frequency = <16000000>;
+	};
+
 	clocks {
 		#address-cells = <1>;
 		#size-cells = <1>;