Documentation: power: Add description about new callback for EM registration

The Energy Model (EM) registration for CPUs should now be done using
a dedicated callback added recently into CPUFreq framework and drivers.

Commit c17495b0 ("cpufreq: Add callback to register with energy model")

The callback guaranties that the EM registration is called at the right
time during driver setup. To avoid mistakes update the documentation
to align with the existing code implementation.

Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>