ARM: at91/dt: add xtal frequencies to sama5d3xcm boards

Define crystal frequencies of sama5d3xcm boards.
Signed-off-by: NBoris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/sama5d3xcm.dtsi

@@ -18,6 +18,14 @@
 		reg = <0x20000000 0x20000000>;
 	};
 
+	slow_xtal {
+		clock-frequency = <32768>;
+	};
+
+	main_xtal {
+		clock-frequency = <12000000>;
+	};
+
 	ahb {
 		apb {
 			spi0: spi@f0004000 {