PM/EM: Document the Energy Model framework

PM/EM: Document the Energy Model framework

Introduce a documentation file summarizing the key design points and
APIs of the newly introduced Energy Model framework.

Signed-off-by: Quentin Perret <quentin.perret@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: corbet@lwn.net
Cc: dietmar.eggemann@arm.com
Cc: morten.rasmussen@arm.com
Cc: patrick.bellasi@arm.com
Cc: qais.yousef@arm.com
Cc: rjw@rjwysocki.net
Link: https://lkml.kernel.org/r/20190110110546.8101-2-quentin.perret@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>