sched/core: uclamp: Update CPU's refcount on clamp changes

Utilization clamp values enforced on a CPU by a task can be updated, for
example via a sched_setattr() syscall, while a task is RUNNABLE on that
CPU. A clamp value change always implies a clamp bucket refcount update
to ensure the new constraints are enforced.

Hook into uclamp_bucket_get() to trigger a CPU refcount syncup, via
uclamp_cpu_{inc,dec}_id(), whenever a task is RUNNABLE.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

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Changes in v6:
 Other:
 - wholesale s/group/bucket/
 - wholesale s/_{get,put}/_{inc,dec}/ to match refcount APIs
 - small documentation updates