sched: Cleanup SCHED_THERMAL_PRESSURE kconfig entry

As Russell pointed out [1], this option is severely lacking in the
documentation department, and figuring out if one has the required
dependencies to benefit from turning it on is not straightforward.

Make it non user-visible, and add a bit of help to it. While at it, make it
depend on CPU_FREQ_THERMAL.

[1]: https://lkml.kernel.org/r/20200603173150.GB1551@shell.armlinux.org.uk



Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200712165917.9168-3-valentin.schneider@arm.com