timekeeping: Remove static functions from kernel-doc markup

Various static functions in the timekeeping code have function comments
which pretend to be kernel-doc, but are incomplete and trigger parser
warnings.

As these functions are local to the timekeeping core code there is no need
to expose them via kernel-doc.

Remove the double star kernel-doc marker and remove excess newlines.

[ tglx: Massaged changelog and removed excess newlines ]

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/1605252275-63652-4-git-send-email-alex.shi@linux.alibaba.com