timekeeping: Rework timekeeping_init() to use shadow_timekeeper

For timekeeping_init() the sequence count write held time is not relevant
and it could keep working on the real timekeeper, but there is no reason to
make it different from other timekeeper updates.

Convert it to operate on the shadow timekeeper.

Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20241009-devel-anna-maria-b4-timers-ptp-timekeeping-v2-19-554456a44a15@linutronix.de