ANDROID: 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.

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Change-Id: I7ad1d6855ac92595377e3334abeb64b158588968
Signed-off-by: Chris Redpath <chris.redpath@arm.com>