ARM: at91/dt: define sama5d3xek's main clk frequency

Define the main clock frequency for the new main clock node
in sama5d3xcm.dtsi.
Signed-off-by: NBoris BREZILLON <b.brezillon@overkiz.com>
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/sama5d3xcm.dtsi

@@ -38,6 +38,12 @@
 			macb0: ethernet@f0028000 {
 				phy-mode = "rgmii";
 			};
+
+			pmc: pmc@fffffc00 {
+				main: mainck {
+					clock-frequency = <12000000>;
+				};
+			};
 		};
 
 		nand0: nand@60000000 {