timekeeping: Introduce timekeeper ID

As long as there is only a single timekeeper, there is no need to clarify
which timekeeper is used. But with the upcoming reusage of the timekeeper
infrastructure for auxiliary clock timekeepers, an ID is required to
differentiate.

Introduce an enum for timekeeper IDs, introduce a field in struct tk_data
to store this timekeeper id and add also initialization. The id struct
field is added at the end of the second cachline, as there is a 4 byte hole
anyway.

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/20250519083025.842476378@linutronix.de

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