cobalt/sched, clock: pipeline: abstract IPI management

The I-pipe and Dovetail share the very same concept of out-of-band,
high-priority IPI, but using a different interface. Let's abstract the
calls manipulating those IPIs to make them pipeline-specific.

No functional change is introduced.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
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