Documentation: DT bindings for energy model cost data required by EAS

EAS (energy aware scheduling) provides the scheduler with an alternative
objective - energy efficiency - as opposed to it's current performance
oriented objectives. EAS relies on a simple platform energy cost model
to guide scheduling decisions. The model only considers the CPU
subsystem.

This patch adds documentation describing DT bindings that should be used to
supply the scheduler with an energy cost model.

Change-Id: I312c8d2f46d3aed0b8f39bd6e4f1739699bc5944
Signed-off-by: Robin Randhawa <robin.randhawa@arm.com>