timekeeping: Add missing parameter docs for pvclock_gtod_[un]register_notifier()

The kernel-doc parser complains about:
 kernel/time/timekeeping.c:651: warning: Function parameter or member
 'nb' not described in 'pvclock_gtod_register_notifier'
 kernel/time/timekeeping.c:670: warning: Function parameter or member
 'nb' not described in 'pvclock_gtod_unregister_notifier'

Add the missing parameter explanations.

[ tglx: Massaged changelog ]

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/1605252275-63652-3-git-send-email-alex.shi@linux.alibaba.com