sched/tune: add detailed documentation

The topic of a single simple power-performance tunable, that is wholly
scheduler centric, and has well defined and predictable properties has
come up on several occasions in the past. With techniques such as a
scheduler driven DVFS, we now have a good framework for implementing
such a tunable.

This patch provides a detailed description of the motivations and design
decisions behind the implementation of the SchedTune.

Change-Id: Ia3c269ecdaab80f1ea8f79a34b795a5c7504a881
cc: Jonathan Corbet <corbet@lwn.net>
cc: linux-doc@vger.kernel.org
Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>