Selecting a Clock Source for the Generic Clock

When changing a generic clock source by writing to CLKCTRL.GEN, the generic clock must be disabled before being re-enabled it with the new clock source setting. This prevents glitches during the transition:

1. Write CLKCTRL.CLKEN=0
2. Assert that CLKCTRL.CLKEN reads '0'
3. Change the source of the generic clock by writing CLKCTRL.GEN
4. Re-enable the generic clock by writing CLKCTRL.CLKEN=1