soc: kwx: Get CPU clock frequency from DTS

The SoC initialization code used system clock frequency
as a CPU clock frequency. This commit corrects that by
obtaining the needed value from DTS.

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

soc/arm/nxp_kinetis/kwx/soc_kw2xd.c

@@ -164,7 +164,7 @@ static ALWAYS_INLINE void clkInit(void)
 	CLOCK_SetSimConfig(&simConfig);
 #if CONFIG_USB_KINETIS
 	CLOCK_EnableUsbfs0Clock(kCLOCK_UsbSrcPll0,
-				CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC);
+				DT_ARM_CORTEX_M4_0_CLOCK_FREQUENCY);
 #endif
 }