time: Introduce auxiliary POSIX clocks

time: Introduce auxiliary POSIX clocks

To support auxiliary timekeeping and the related user space interfaces,
it's required to define a clock ID range for them.

Reserve 8 auxiliary clock IDs after the regular timekeeping clock ID space.

This is the maximum number of auxiliary clocks the kernel can support. The actual
number of supported clocks depends obviously on the presence of related devices
and might be constraint by the available VDSO space.

Add the corresponding timekeeper IDs as well.

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/20250519083025.905800695@linutronix.de