timekeeping: Fix kerneldoc system_device_crosststamp & al

Make kernel doc comments actually work and fix the syncronized typo.

[ tglx: Added the missing /** and fixed up formatting ]

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200609081726.5657-1-kurt@linutronix.de