sched: Documentation for scheduler energy cost model

This documentation patch provides an overview of the experimental
scheduler energy costing model, associated data structures, and a
reference recipe on how platforms can be characterized to derive energy
models.

Change-Id: I660906d9e23f79e2e0c4a0459c63ade62c61c4fa
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>