ARM: at91/dt: define main clk frequency of at91sam9rlek

Define the main clock frequency for the new main clock node in
at91sam9rlek.dts

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris BREZILLON <b.brezillon.dev@gmail.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/at91sam9rlek.dts

@@ -105,6 +105,12 @@
 				};
 			};
 
+			pmc: pmc@fffffc00 {
+				main: mainck {
+					clock-frequency = <12000000>;
+				};
+			};
+
 			watchdog@fffffd40 {
 				status = "okay";
 			};