dts: Add information about CPU frequency to the cpu nodes

This commit adds 'clock-frequency' property to the cpu nodes.
The clock frequency specified in the added property is used
during platform configuration. Examples:

- The SWO logger uses clock frequency to configure SWO output.
- Plenty of platforms need CPU clock specified for their HAL.
- Most of devices with USB needs information about CPU clock
  in order to configure USB clock source.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>

boards/arm/frdm_kl25z/frdm_kl25z.dts

@@ -61,6 +61,10 @@
 	};
 };
 
+&cpu0 {
+	clock-frequency = <48000000>;
+};
+
 &adc0 {
 	status = "okay";
 };