ARM: at91/dt: define sam9261ek slow crystal frequency

Define at91sam9261ek's slow crystal frequencies.
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/at91sam9261ek.dts

@@ -20,6 +20,10 @@ memory {
 		reg = <0x20000000 0x4000000>;
 	};
 
+	slow_xtal {
+		clock-frequency = <32768>;
+	};
+
 	main_xtal {
 		clock-frequency = <18432000>;
 	};