timekeeping: Provide time getters for auxiliary clocks

timekeeping: Provide time getters for auxiliary clocks

Provide interfaces similar to the ktime_get*() family which provide access
to the auxiliary clocks.

These interfaces have a boolean return value, which indicates whether the
accessed clock is valid or not.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20250625183757.868342628@linutronix.de