ARM: at91/dt: add xtal frequencies to sama5d3 xplained board

Define crystal properties of sama5d3 xplained board.
Signed-off-by: NBoris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/at91-sama5d3_xplained.dts

@@ -21,6 +21,14 @@
 		reg = <0x20000000 0x10000000>;
 	};
 
+	slow_xtal {
+		clock-frequency = <32768>;
+	};
+
+	main_xtal {
+		clock-frequency = <12000000>;
+	};
+
 	ahb {
 		apb {
 			mmc0: mmc@f0000000 {