ARM: at91/dt: aks-cdu: define slow crytal frequency

Define AK signal CDU slow crystal frequency
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: NNicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/aks-cdu.dts

@@ -16,6 +16,12 @@
 		bootargs = "console=ttyS0,115200 ubi.mtd=4 root=ubi0:rootfs rootfstype=ubifs";
 	};
 
+	clocks {
+		slow_xtal {
+			clock-frequency = <32768>;
+		};
+	};
+
 	ahb {
 		apb {
 			usart0: serial@fffb0000 {