SystemCoreClockUpdate - wrong implementation

[0xc0170]

As mentioned in the pull request #1848, some targets do not conform SystemCoreClockUpdate (). they just return the predefined value.

The api docs say:

Updates the variable SystemCoreClock and must be called whenever the core clock is changed during program execution. The function evaluates the clock register settings and calculates the current core clock.

We should review all the implementation, create a list of targets that need an update.

[ciarmcom]

ARM Internal Ref: IOTMORF-167