Documentation: EM: Update with runtime modification design

Add a new description which covers the information about runtime EM.
It contains the design decisions, describes models and how they
reflect the reality. Remove description of the default EM. Add example
driver code which modifies EM. Add API documentation for the new feature
which allows to modify the EM in runtime.

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>