sched/core: uclamp: Add system default clamps

Tasks without a user-defined clamp value are considered not clamped
and by default their utilization can have any value in the
[0..SCHED_CAPACITY_SCALE] range.

Tasks with a user-defined clamp value are allowed to request any value
in that range, and we unconditionally enforce the required clamps.
However, a "System Management Software" could be interested in limiting
the range of clamp values allowed for all tasks.

Add a privileged interface to define a system default configuration via:

  /proc/sys/kernel/sched_uclamp_util_{min,max}

which works as an unconditional clamp range restriction for all tasks.

If a task specific value is not compliant with the system default range,
it will be forced to the corresponding system default value.

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

---
The current restriction could be too aggressive since, for example if a
task has a util_min which is higher then the system default max, it
will be forced to the system default min unconditionally.

Let say we have:

   Task Clamp:    min=30, max=40
   System Clamps: min=10, max=20

In principle we should set the task's min=20, since the system allows
boosts up to 20%. In the current implementation, however, since the task
mins exceed the system max, we just go for task min=10.

We should probably better restrict util_min to the maximum system
default value, but that would make the code more complex since it
required to track a cross clamp_id dependency.
Let's keep this as a possible future extension whenever we should really
see the need for it.

Changes in v6:
 Others:
 - wholesale s/group/bucket/
 - make use of the bit_for() macro