SystemCoreClockUpdate() not implemented for MCXN947

[TheoBrunner]

Before starting the Keil RTX5 RTOS, the SystemCoreClock variable must be updated using the SystemCoreClockUpdate() function to ensure the SysTick timer is initialized correctly. However, for the MCXN947 MCU, the implementation of SystemCoreClockUpdate() in devices/MCXN947/system_MCXN947_cm33_core0.c is empty.

This behavior seems unintended, as it prevents the SystemCoreClock from reflecting the actual system clock frequency, which is essential for correct RTOS operation.